Lighted Incubation in Poultry: Current Insights and Prospects for Enhancing Embryonic and Post-Hatch Development

Poultry productivity is closely tied to embryonic and post-hatch development, making optimal incubation conditions fundamental for achieving desirable performance outcomes. Post-hatch growth, skeletal muscle development, and meat quality depend significantly on both egg storage conditions and incubation parameters, such as relative humidity, temperature, ventilation, lighting, egg turning, delayed access to feed, in ovo feeding, and oxygen density (Tona et al., 2022; Wang et al., 2022). Egg fertility and management of both setters and hatcheries can also affect hatchability (Araújo et al., 2016).

In addition to specific incubation conditions, hatchery type has a considerable impact on hatchability and post-hatch outcomes. Multistage and single-stage incubators are commonly used in commercial settings. Multistage systems tend to exhibit higher embryo mortality, more live and dead pipped chicks, and a greater number of cracked eggs, partly due to transportation-related stress (Mesquita et al., 2021). The increased mortality in multistage incubators is often attributed to elevated temperatures caused by heat transfer from older to younger embryos as the system attempts to maintain thermal balance (Araújo et al., 2016). Despite these drawbacks, multistage machines reduce overall incubation energy consumption because they rely on embryo-derived heat, making them relatively cost-efficient. Conversely, single-stage incubators typically provide higher hatchability, a more uniform hatch window, and better chick physical quality (Araújo et al., 2016; Mesquita et al., 2021). Because all eggs are at the same developmental stage, incubation conditions can be precisely adjusted to the embryo’s requirements (Araújo et al., 2016). In addition, single-stage units can be completely emptied after each hatch, allowing for thorough cleaning and disinfection and thus improving sanitation (Mesquita et al., 2021).

Achieving optimal hatchability requires meeting all critical incubation conditions, particularly temperature and lighting, regardless of hatchery type. Incubation temperature and lighting play central roles in embryonic development and post-hatch physiology. For example, pineal aralkylamine N-acetyltransferase, the rate-limiting enzyme in melatonin synthesis, is regulated by both light exposure and temperature (Yalcin et al., 2022). The optimal incubation temperature for maximizing hatchability is 37.5-37.8 ºC. Wijnen et al. (2020) reported that an eggshell temperature of 38.9 ºC during week 2 increased post-hatch chick length and lowered blood glucose compared to 37.8 ºC, whereas an eggshell temperature of 36.7 ºC during the third week reduced post-hatch chick length, body weight, average daily gain, and feed intake, and increased blood glucose, heart, and stomach weight.

Lighting during incubation influences the intra-egg environment and shapes post-natal physiological responses. Permanent darkness acts as a stressor, reducing hatchability and impairing post-hatch performance (Riaz et al., 2024). Photostimulation can affect hatchability, the hatch window, the percentage of fertile eggs that hatch, post-hatch growth, chick quality, stress susceptibility, welfare indicators, and economic outcomes (Seremet and Bayraktar, 2021; Riaz et al., 2024).

Incubation lighting also modulates embryonic development, physiological traits, stress responses, fear-related behavior, spatial abilities, and cognitive performance (Yalcin et al., 2022; Riaz et al., 2024). Furthermore, embryonic hemispheric lateralization, circadian entrainment, and establishment of melatonin rhythm (key factors influencing thermoregulation) are all dependent on light exposure (Yalcin et al., 2022). Intermittent light for 12 hours per day improves hatching traits, meat quality, and blood biochemical parameters (Yameen et al., 2020). White light-emitting diodes (LED; 5000K; 250 lux) provided for 12 or 24 h during incubation enhances hatchability, narrows the hatch window, increases the hatch-to-fertile percentage, and improves carcass yield (Riaz et al., 2021). Similarly, 12 hours of LED light at 250 lux reduces embryonic mortality, the incidence of dead germs and dead-in-shell embryos, as well as improving feed conversion ratio (FCR) and stress susceptibility indicators such as physical asymmetry and heterophil-to-lymphocyte (H/L) ratio (Riaz et al., 2024).

Light wavelength also affects hatching and post-hatching performance, as avian embryos respond sensitively to spectral differences. Approximately 98.2% of incident light is absorbed by the eggshell, and only a small fraction reaches the embryo (Yameen et al., 2020). In avian species, stimulation of the light-sensitive pineal gland enhances embryonic growth and shortens incubation time when exposed to certain wavelengths and intensities. Embryonic exposure to light promotes post-hatch performance and welfare, although high light intensity may decrease chick weight (Tainika and Bayraktar, 2022). Green light during incubation enhances muscle development and weight gain, whereas red and white light generally improve hatchability (Tainika and Bayraktar, 2022). Incubation light wavelengths significantly shape development. Red light exposure during the dark phase (scotophase) enhances immune health lowering air cell temperature while boosting melatonin secretion, IgG levels, and the expression of immune genes like AvBD-1 and IL-6. Conversely, white or blue light prioritizes physical growth, leading to higher feed consumption and increased body weight (Li et al., 2021a, b; Kankova et al., 2022). Green light reduces serotonin concentration, increases age-related pecking behavior, and upregulates hypothalamic 5-HTT expression, whereas white light increases CRH, 5-HTR1A, and 5-HTR1B expression (Özkan et al., 2022). Green light exposure for 12 hours shortens incubation time, increases serum insulin-like growth factor 1 (IGF-1), increases relative liver weight, and modifies pineal gland follicle size and structure. It also alters arylalkylamine N-acetyltransferase (AANAT), brain/muscle ARNT-like-1 (BMAL1), circadian locomotor output cycles kaput (CLOCK), and cryptochrome-1 (CRY1) expression (Wang et al., 2021). Providing green LED light at 0.1 W/m2 from day 18 to hatch increases somatotropic axis expression (Dishon et al., 2021).

Beyond wavelength, light intensity, and specific light combinations exert distinct effects. In quail, 250 lux of green light reduces embryonic mortality and improves chick spread, growth performance, pre-slaughter weight, and carcass weight, compared with 150 or 350 lux (Ali et al., 2023). A combination of green and red light at 250 lux enhances hatchability, late-stage embryonic survival, chick spread, growth performance, serum chemistry, and carcass traits relative to a green-blue or blue-red combination (Safwan et al., 2023). Although many studies report positive effects of incubation lighting on hatchability and post-hatch performance, findings are not fully consistent. For example, Riaz et al. (2024) reported no significant effects of white, green, or red LEDs on pre-hatch development, post-hatch growth, production traits, or welfare. However, the inconsistency in literature regarding the effects of illuminated incubation comes down to a complex web of biological, environmental, and methodological variables that differ from one study to another.

This review aims to synthesize current evidence on LED lighting, spectral wavelengths, and intensities during incubation to clarify optimal lighting conditions for maximizing hatchability and post-hatch performance. To ensure a comprehensive and unbiased synthesis of the existing evidence, a structured literature search was conducted prior to drafting this review. Scientific articles were identified through major academic databases, including PubMed, Web of Science, Scopus, and Google Scholar. The systematic search targeted original research articles published between 2000 and 2025. However, classic foundational studies, international technical guides (e.g., FAO, 2004), and essential textbook or review sources were manually retrieved and included to provide necessary physiological, historical, and methodological context, using combinations of the following keywords and Boolean operators: lighted incubation, avian embryo, poultry incubation lighting, LED incubation, embryonic development, hatchability, photostimulation, light wavelength, and post-hatch performance. Only peer-reviewed original research articles written in English were considered eligible. Reference lists of relevant publications were also screened manually to identify additional studies not captured in the database searches. Studies were included: (i) the effects of light exposure during incubation on embryonic development, hatchability, physiology, behavior, or post-hatch performance; or (ii) the influence of specific lighting parameters such as wavelength, intensity, photoperiod, or LED technology. Exclusion criteria included: studies not involving avian species, papers lacking experimental data (e.g., opinion pieces), conference abstracts without full text, and articles not providing clear incubation lighting protocols. No restrictions were applied regarding bird species, study design, or type of lighting system to ensure broad coverage of current knowledge.

Avian embryogenesis is a complex process starting from fertilization to hatching (Yahav and Brake, 2014). Birds are oviparous, and fertilization within the female reproductive tract initiates embryonic development. A freshly laid egg exhibits low internal oxygen tension and depends on external heat to support development, but it contains most of the structural and nutritional components required for embryogenesis (Mueller et al., 2022). Atmospheric oxygen diffuses through microscopic pores in the eggshell, while water vapor and CO2 produced by the embryo diffuse outward (Lock et al., 2024). Incubation typically ranges from 21 to 42 days, depending on the species (FAO, 2004); it may occur naturally or artificially, with artificial incubation providing controlled conditions from oviposition to hatching (Abd El-Hack et al., 2022). Optimal artificial incubation conditions generally include 36 - 39 °C (species-dependent), 50-65 % relative humidity, an airflow of 0.4 m/s, and hourly egg turning to prevent adhesion of the embryo to the eggshell (Kosemani et al., 2025; Leaphart et al., 2025).

Key developmental milestones include blastoderm formation, organogenesis, and establishment of the circulatory system, which supports nutrient distribution and waste removal (Ribatti, 2022). Following fertilization, rapid cleavage produces a blastoderm that differentiates into three germ layers: ectoderm, mesoderm, and endoderm, the germ layers that give rise to all tissues and organs (Benchikh, 2024). Subsequently, organogenesis proceeds, the heart begins to beat, and the major systems, such as the musculoskeletal, nervous, and digestive systems, begin to take shape (Muhr et al., 2023). The yolk provides the primary nutrients and energy for embryonic development (Blackburn and Stewart, 2021), while albumen serves as an additional nutritional reservoir, and the shell and membranes regulate water loss and gas exchange. During early life, oviparous vertebrates are particularly vulnerable to environmental stressors due to limited parental protection (especially in poultry) and wide variability in incubation environments (Lock et al., 2024).

Over the last decade, many studies have examined the effect of slight deviations from optimal incubation temperature (±1-1.5 °C) on embryonic development (Yalcin et al., 2022). Both elevated and reduced temperatures impair yolk sac absorption and yolk nutrient utilization by altering the expression of genes involved in digestion and absorption, ultimately compromising chick quality (Dayan et al., 2020). Moderate temperature fluctuations during embryonic development resulted in reduced hatchability, increased day-old chick (DOC) weight, and delayed hatching. Temperatures of 36-36.5°C on day 15 prolong external pipping and delay emergence, whereas early-stage exposure to 38.1°C reduces DOC weight and yolk sac mass, and increases chick length (Lin et al., 2017). Exposure to 38.6°C further decreases hatchability (Avşar et al., 2022). Conversely, Wijnen et al. (2020) showed that exposure to 38.9°C during week 2 accelerates embryo development, shortens the hatch window, and reduces incubation duration with no effect on DOC weight. Muir and Groves (2018) observed higher hatchability, more late-hatch chicks, and increased bone ash when eggs were incubated at 37.2°C to 37.8°C for 13 days.

Along with temperature, humidity is also a key factor. Water loss during incubation must fall within an optimal range; losses below 9.1% or above 18.5% increase mortality (Lin et al., 2024). A relative humidity between 40 and 70% is generally recommended. Low humidity reduces chick body weight and can harden the chorion, making hatching difficult and lowering hatchability (Bruzual et al., 2000). Excessive humidity leads to early hatching and poor chick quality (Peebles, 2023). Ventilation is equally essential for regulating O2 uptake and CO2 removal; inadequate ventilation creates hypoxic and hypercapnic conditions detrimental to the embryo (Juá et al., 2024). Adequate O2 levels are vital for embryonic metabolic pathways; deficiencies induce hypoxia, which suppresses metabolic rates and impairs energy production due to low breathing rates and hinders cell proliferation, differentiation, and growth. In the case of artificial incubation, proper ventilation with an O2 level of 21% to 23% in the setter is recommended, with a lower CO2 level, as a high level of CO2 causes respiratory acidosis and disruption of acid-base balance in the developing embryo and may lead to embryo death (Juá et al., 2024).

In the last decade, interest has increased regarding the role of light during avian embryonic development. Light exposure can influence physiology, post-hatch behavior, growth, cognitive ability, stress resilience, and fear responses (de Haas, 2020). These effects may be mediated by hemisphere lateralization and the entrainment of circadian rhythms before hatching (Yalcin et al., 2022). Although timing, duration, intensity, and spectral composition of light exposure exert significant effects during embryogenesis, there is still a need to deeply understand the effect of light on embryo development.

Light exposure during embryonic development significantly influences hatching performance, embryonic growth, stress responsiveness, welfare, behavior, and overall post-hatch performance in poultry species (Yalcin et al., 2022). Avian embryos can perceive light as early as 2 days of incubation, with fully functional photoreception established by embryonic day (ED) 18 (Yalcin et al., 2022; Oso et al., 2023). Light perception during this period is mediated not only by retinal photoreceptors but also by extraretinal photoreceptors located in the pineal gland and hypothalamus, specifically within the paraventricular and the lateral septal areas (Surbhi and Kumar, 2015). These hypothalamic photoreceptors, commonly referred to as deep brain photoreceptors (DBPs), allow embryos to respond to light stimuli even before the visual system is fully developed (Yalcin et al., 2022).

Retinal and pineal photoreceptors, activated according to the photoperiod, contribute to circadian regulation of physiology and behavior through melatonin secretion, whereas DBPs are implicated in longer-term seasonal and developmental regulation, including gonadal maturation and growth responses to photoperiodic cues (Surbhi and Kumar, 2015). These photoreceptors express a variety of opsins capable of detecting specific wavelengths and converting light signals into electrochemical impulses propagated through neural circuits to regulate downstream physiological pathways (Lee et al., 2021). Opsins expressed in the hypothalamus have been associated with regulation of the major endocrine axes: the hypothalamic-pituitary-adrenal (HPA), hypothalamic-pituitary-gonadal (HPG), and hypothalamic-pituitary-thyroid (HPT) axes (Özkan et al., 2022). Moreover, light exposure can influence embryonic neurological development from as early as embryonic day 3 (Sampaio et al., 2024). The embryonic response to light is therefore dependent on the timing, intensity, and spectral composition of the light provided.

Light plays a central role in regulating the embryonic circadian rhythm, particularly through its effects on the pineal gland. Early exposure to light synchronizes the circadian clock by modulating pineal gene expression and establishing rhythmic melatonin secretion (Li et al., 2024a). The presence of light suppresses melatonin synthesis by inhibiting the enzyme arylalkylamine N-acetyltransferase (AANAT), which converts serotonin into melatonin. Conversely, during darkness, norepinephrine is released at pineal synapses, activating AANAT through phosphorylation of the cyclic adenosine monophosphate(cAMP)-responsive element-binding protein (CREB), ultimately increasing melatonin synthesis (Horodincu and Solcan, 2023). Photoreception is further mediated by the cAMP and cyclic guanosine monophosphate (cGMP), signaling pathways, which regulate circadian gene expression (e.g., CRY2) and microRNAs (e.g., gga-miR-34b-5p) essential for maintaining circadian rhythmicity (Li et al., 2024b).

In the retina, melanopsin (Opn4) within intrinsically photosensitive retinal ganglion cells detects light and transmits circadian information to the brain (Pan et al., 2025). Although only 0.5 to 5% of these cells are intrinsically photosensitive, they generate electrical signals that are relayed to the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract (RHT) (Ashton et al., 2022; Guarnieri, 2024). The SCN, acting as “master clock”, regulates circadian rhythms and influences downstream physiological systems, including the HPA axis, thereby modulating metabolic regulation, feeding behavior, and endocrine activity (Gu et al., 2021; Li et al., 2024a). Although the RHT is not fully developed in early embryonic stages, melanopsin-expressing retinal cells are still believed to mediate light detection and circadian entrainment.

During the early embryonic stage, DBPs play a more predominant role than the retina in photoreception. Opsins, such as encephalopsin (Opn3), neuropsin (Opn5), and vertebrate ancient opsin (VA opsin), are expressed in the paraventricular organ (PVO) and other brain regions and enable the embryo to detect light independently of the eyes (Schöneberg, 2025). These opsins activate G-protein-coupled receptors (GPCRs), triggering intracellular signaling cascades (Yee et al., 2023). GPCRs activation leads to dissociation of G-protein subunits and initiation of the key pathways, including the phosphoinositide (PLC) and cAMP pathways (Chung and Wong, 2021; Li et al., 2024b). In the PLC pathway, hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) produces inositol trisphosphate (IP3) and diacylglycerol (DAG), promoting intracellular calcium release and protein kinase C (PKC) activation (Özkan et al., 2022; Liccardo et al., 2022). On the other hand, cAMP pathways activate protein kinase A (PKA) through adenylate cyclase-mediated conversion of ATP to cAMP (Chung and Wong, 2021). Both pathways lead to phosphorylation of transcription factors like cAMP and CREB, regulating circadian gene transcription, including CLOCK, BMAL1, PER, and CRY (Ashton et al., 2022). Through these mechanisms, avian embryos synchronize their physiological processes with environmental light cues, influencing developmental trajectories that extend into post-hatch life.

A growing body of evidence demonstrates that light exposure during incubation profoundly affects embryonic physiology, developmental trajectories, and hatching outcomes. The magnitude and direction of these effects depend on light wavelength, intensity, duration, and photoperiod, with green and blue wavelengths generally producing the most consistent improvements in embryonic growth, hatch timing, and physiological maturation, as summarized in Table 1.

Table 1: Light’s effects on the physiology, growth, and behavior of avian embryos during the incubation period.

The influence of incubation lighting on hatching performance is fundamentally driven by the activation of non-visual photoreceptors specifically inside the avian embryonic pineal gland and the retina which are responsive to light stimulation as early as the first week of incubation (Wang et al., 2021). Photoperiodic light exposure, particularly in the green and white spectra, upregulates the activity of the embryonic somatotropic axis, stimulating the expression of growth hormone and insulin-like growth factors (Dishon et al., 2017). This hormonal surge accelerates embryonic metabolism, cellular proliferation, and nutrient utilization from the yolk sac (Dayan et al., 2020). Furthermore, light stimulation enhances the rhythmic secretion of melatonin and activates the circadian clock network, which plays a pivotal role in synchronizing the developmental rate of the embryos within the incubator (Oso et al., 2023; Li et al., 2024b). On a physical level, this advanced physiological maturity promotes stronger embryonic muscle development and increases late-stage pipping activity, ultimately narrowing the hatch window and improving overall hatchability rates without compromising post-hatch chick quality (Tainika and Bayraktar, 2022; Yin et al., 2024).

Light exposure consistently improves several morphological traits. Abdulateef et al. (2021) reported significantly higher embryo weight on day 19 in eggs incubated under red (33.55 g), blue (33.93 g), and green light (34.70 g) compared with complete darkness (30.00 g; p < 0.01). Their study also revealed a substantial enhancement in hatchability under green light (77.0%), which was significantly higher than blue (72.0%), red (69.0%), and dark incubation (66.7%). The same authors documented marked improvements in neurophysiological traits under photostimulation: Embryos incubated under green and blue light exhibited significantly larger neuron diameters (45.63 μm and 43.03 μm, respectively) and heavier brain weights compared with red light and dark incubation, suggesting wavelength-dependent modulation of neural development. These findings align with the notion that green and blue wavelengths penetrate the eggshell more effectively and exert a stronger influence on the central nervous system during critical periods of neuronal growth.

The stimulatory effects of green light on somatic development have been repeatedly confirmed. Rozenboim et al. (2004) demonstrated that green photostimulation increased yolk temperature in a time-dependent manner, with each minute of exposure raising yolk temperature by approximately 0.01% of baseline. Although continuous exposure caused persistent temperature elevations, an intermittent cycle (15 min light/ 15 min dark) prevented excessive thermal accumulation while preserving developmental benefits. Embryos incubated under intermittent green light showed higher body weight on embryonic days 14, 15, 17, and 20, together with a sustained increase in pectoralis major muscle percentage from day 9 to day 21 relative to dark incubation. These results reinforce the hypothesis that green light enhances muscle accretion via mechanisms linked to the somatotropic axis.

Other studies corroborate the positive influence of green light on hatchability. Yameen et al. (2020) observed that eggs exposed to 12 h of light exhibited higher hatchability compared with those incubated in continuous darkness, along with reduced late embryonic mortality. Although 24 h of light increased moisture loss and altered mortality patterns, it nevertheless improved certain hatch window dynamics, with light-stimulated chicks emerging earlier during specific pre-hatch intervals. Differences among strains were also reported, highlighting the importance of genotype in modulating embryonic responses to photoperiod. Notably, eggs incubated with 12 h of light produced heavier chicks at hatch, while continuous light reduced chick weight at pull-time, suggesting a non-linear relationship between photoperiod and energy utilization during development. Riaz et al. (2024) similarly reported higher hatchability and hatch-of-fertile percentages under both 12 h and 24 h photoperiods compared with dark incubation, together with narrower hatch windows. Importantly, illuminated embryos exhibited lower heterophil-to-lymphocyte ratios and reduced physical asymmetry, indicating an improved pre-hatch stress response. These findings reinforce the concept that embryonic photostimulation not only accelerates developmental processes but also supports improved physiological resilience at hatch.

The influence of lighting on endocrine regulation during late embryogenesis has been explored in detail by Tong et al. (2018), who compared illuminated and dark-incubated embryos under identical incubation conditions. Although overall hatchability of the focal chicks was higher in the control group when considering cumulative mortality, photostimulation advanced the mean hatch time by approximately 3.4 h, with a larger proportion of focal chicks hatching earlier than their dark-incubated counterparts. Hormonal analyses revealed that light exposure modified the temporal trajectories of melatonin (MT), corticosterone (CORT), and thyroid hormones (T3 and T4) during the 12:12 h light–dark cycle. While absolute concentrations were not consistently different between treatments at individual time points, light altered the rhythmic fluctuations of these hormones. For example, MT in the dark group sharply increased during the first 3 h of scotophase, whereas the light group showed a more gradual rise with a peak at 9 h. Similarly, T3 levels in light-exposed embryos were significantly higher during the illuminated period (p= 0.02), suggesting that photostimulation may facilitate earlier maturation of metabolic pathways. These endocrine modulations, although subtle, may contribute to the accelerated hatching observed in illuminated embryos.

Importantly, not all studies have reported improvements in hatchability. Drozdová et al. (2021), for instance, found no significant differences in hatchability among red, white, green, and blue light treatments, despite reporting wavelength-specific effects on pineal melatonin synthesis. This inconsistency highlights the complexity of the system and suggests that experimental factors such as eggshell properties, breeder flock age, light intensity, and thermal environment may interact with photostimulation in determining hatchability outcomes.

Taken together, the available evidence indicates that embryonic photostimulation consistently enhances growth trajectories and developmental maturity, particularly under green and blue wavelengths. However, improvements in hatchability are not universally observed and may depend on species, strain, photoperiod, and incubation conditions. The variability across studies underscores the necessity for standardized protocols that consider both the physiological sensitivity of the embryo to specific wavelengths and the potential interactions between lighting, temperature, and humidity.

The enhancement of post-hatch growth performance via lighted incubation is primarily driven by the photostimulation of the embryonic somatotropic axis and the acceleration of muscle development. In ovo photoperiodic exposure particularly using green or white monochromatic light stimulates embryonic photoreceptors, triggering intracellular cascades that upregulate the expression of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) genes (Dishon et al., 2017; Wang et al., 2021). This prenatal activation of the somatotropic axis enhances yolk sac nutrient absorption and energy utilization during the late incubation stage, providing a robust metabolic foundation for post-hatch vitality (Dayan et al., 2020; Lin et al., 2024). Furthermore, specific green light wavelengths penetrate the eggshell to stimulate myofiber hyperplasia by accelerating the proliferation and differentiation of muscle satellite cells during embryogenesis (Rozenboim et al., 2004; Wang et al., 2022). Consequently, chicks incubated under appropriate light regimes exhibit superior post-hatch feed consumption, efficient muscle accretion, and significantly higher early body weight gain compared to those incubated in continuous darkness (Riaz et al., 2021; Li et al., 2021b).

Light exposure during incubation exerts marked effects on growth performance, physiological maturation, and carcass traits of poultry, although the magnitude and direction of these responses depend strongly on photoperiod, wavelength, and light intensity. Several studies indicate that appropriate photostimulation enhances feed efficiency and carcass traits without compromising general livability (Table 2). Riaz et al. (2021) reported that chicks incubated under 12 hours of light displayed significantly improved FCR compared with both 24 h light and dark incubation, despite the absence of differences in feed intake, body weight gain, or livability. Carcass yield also improved under illuminated incubation (Riaz et al., 2021), whereas leg quarter yield was higher in chicks originating from dark and 24 h light incubation compared with the 12 h group, suggesting that sustained or absent light may differentially direct nutrient partitioning during embryogenesis (Riaz et al., 2021). Studies involving different breeds corroborate this selective sensitivity. Yameen et al. (2020) found that 12 h light incubation produced chicks with significantly higher initial body weight than those incubated in the dark, while 24 h illumination produced the lowest chick weight.

Table 2: Light and its effect during incubation on different species of poultry.

Although chicks incubated in darkness showed the highest post-hatch feed intake, body weight gain, and livability, those exposed to 12 h light retained the most favorable FCR, supporting the hypothesis that moderate photoperiods facilitate more efficient metabolic programming. Strain differences further influenced these outcomes: Hubbard chicks exhibited higher initial chick weight, whereas Ross and Hubbard displayed superior body weight and FCR compared with Cobb strains (Yameen et al., 2020). These genotype-dependent interactions emphasize the need for tailored lighting strategies for specific commercial breeds.

Beyond gross performance, light exposure modulates organ development and endocrine profiles. White light at 200 lux in Beijing-You chicken eggs improved long bone development, antioxidant status, and early leg health (Geng et al., 2021), although body weight and several organ weights at hatch remained unaffected. Nevertheless, relative brain weight approached significance (p= 0.052), suggesting potential neural sensitivity to photoperiod. Similarly, Abdulateef et al. (2021) observed that embryos incubated under green or blue light exhibited higher 1-day-old body weight than those from dark or red-light conditions, highlighting wavelength-specific neuroendocrine effects.

Current research in ornithological science indicates that light sources and wavelengths profoundly affect bird performance, welfare, and physiology (Soliman et al., 2023; Abdel-Moneim et al., 2024). Dishon et al. (2017) demonstrated that intermittent green light (15 min cycles at 0.1 W/m2) significantly increased hypothalamic growth hormone-releasing hormone (GHRH) expression and hepatic growth hormone receptor abundance. Yu et al. (2018) reported that exposure to 50 lux green light for 16 h not only enhanced hatchling body weight and length but also elevated circulating T4 and testosterone concentrations, hormones strongly associated with growth acceleration. Red-light incubation has also been shown to improve body weight during days 18-21 post-hatch compared with blue light (Drozdová et al., 2021), underscoring the nuanced interactions between wavelength and physiological programming.

Welfare-related traits and behavioral development are likewise strongly influenced by embryonic photostimulation. Early hemispheric lateralization induced by light exposure has been linked to improved attentional control, visual discrimination, and reduced fear responses. Chiandetti et al. (2013) observed that pullets incubated under incandescent light accessed feeders more efficiently than those incubated in darkness. Green-light incubation enhanced approach behavior towards humans at 400 lux (Kliphuis et al., 2024), whereas full-spectrum light at 550 lux increased feeding motivation shortly after dawn in broilers (Archer and Mench, 2014). Light also modulated neuroendocrine stress markers: green, red, and white light at 250 lux significantly increased plasma serotonin while reducing corticosterone in hatchlings compared with dark incubation (Archer, 2017). Behavioral resilience tests further confirmed these effects, with light-incubated chicks showing faster emergence, shorter tonic immobility, and reduced wing flapping during inversion compared with dark-incubated controls (Archer and Mench, 2014).

Post-hatch musculoskeletal traits and leg health appear responsive to certain light regimens. Incubation under white light at 500 lux improved broiler leg health compared with dark incubation or lower-intensity schedules (van der Pol et al., 2019). However, studies differ on the extent to which photostimulation impacts bone morphology or gait score. While Riaz et al. (2021) found no differences in gait or feather score at day 35, Yameen et al. (2020) reported that chicks from 12 h and 24 h light incubation exhibited higher feather scores on day 15, though these differences disappeared by day 35. Physical asymmetry, an indicator of developmental instability, was highest in dark-incubated birds and lowest in 24 h light-exposed chicks, suggesting that continuous light may stabilize certain developmental processes even when other traits are negatively affected.

It was observed that green, red, and white light at 250 lux during incubation of broiler eggs led to chicks with significantly higher plasma serotonin and lower plasma corticosterone concentrations than the chicks incubated in the dark, ultimately lowering fear and stress susceptibility (Archer, 2017). Similarly, broiler birds incubated under fluorescent light at 550 lux throughout the incubation quickly left the box during the emergence test, recovered quickly during the tonic immobility test, and flapped less intensely during the inversion test than the birds incubated under 12 hours of light given on the 14th day and the dark group. In the same study by Archer and Mench (2014), the broiler birds incubated under light vocalized the least during the isolation test compared to birds incubated in the dark.

Carcass composition and meat quality traits are also modulated by light exposure. Yameen et al. (2020) documented that breast and thigh yields were significantly higher in chicks from 12 h light incubation, whereas abdominal fat was elevated in 24 h light–incubated birds. Carcass yield was greater in 0 h and 12 h light treatments than in 24 h, reinforcing the detrimental effect of continuous illumination on nutrient distribution. Meat quality traits responded selectively: shear force was higher in progeny from dark and 12 h light incubation, while color parameters (L*, a*, b*), a standardized color measurement system, varied with light duration and strain. Notably, meat redness at 24 h post-slaughter was significantly higher in the 12 h light group (Yameen et al., 2020), an indicator often associated with better consumer acceptability.

Collectively, these findings indicate that intermediate photoperiods (particularly 12L:12D), moderate intensities (200-500 lux), and specific wavelengths (especially green light) consistently produce favorable growth performance, endocrine profiles, welfare indicators, and carcass characteristics. Conversely, continuous light (24 h) often impairs chick quality, reduces hatchling weight, increases late embryonic mortality, and contributes to excessive abdominal fat deposition. Dark incubation, while promoting feed intake and livability in some contexts, generally results in inferior FCR, impaired behavioral development, and heightened stress sensitivity.

The integration of lighted incubation protocols into commercial hatchery systems presents both promising opportunities and substantial practical challenges. Although a large body of experimental research indicates that appropriate light stimulation can enhance embryonic development, improve hatchability, modulate endocrine and circadian programming, and promote favorable behavioral and welfare outcomes, the poultry industry still largely relies on dark incubation. This conservative approach stems from long-standing concerns regarding potential reductions in hatchability, increased energy expenditure associated with unwanted secondary heating, and the possibility that non-optimized lighting parameters may compromise chick performance rather than enhance it (Rozenboim et al., 2003). One of the major constraints limiting widespread adoption is the absence of standardized and universally applicable guidelines for light wavelength, photoperiod, and intensity across breeds and production types. Empirical studies have demonstrated that responses to light are influenced not only by the spectral properties of the source but also by species- and strain-specific differences, incubation equipment, shell pigmentation, and even parental flock age (Li et al., 2021a, b). As a consequence, protocols that are beneficial in one genetic line or management system do not necessarily yield the same results elsewhere, creating uncertainty for industry stakeholders. Another practical limitation involves the interaction between lighting and thermal regulation.

Several studies have shown that continuous or prolonged photostimulation, particularly with short wavelengths such as green light, can incrementally increase yolk temperature, potentially shifting the metabolic heat production profile of the embryo. In large-scale commercial setters, which operate with narrow thermal margins, even minor temperature deviations may translate into significant variation in hatch windows and chick quality if not carefully controlled. This necessitates the integration of lighting technologies that minimize heat emission, alongside ventilation and thermal compensation systems capable of counterbalancing subtle metabolic changes (Yin et al., 2024; Iwuchukwu et al., 2025). Light distribution within commercial incubators also poses technical challenges. Achieving uniform exposure across a high-density tray layout is difficult because eggs located at different positions within the setter receive unequal light intensities. Variability in shell thickness and pigmentation further affects light penetration, leading to inconsistent stimulation among embryos. Advanced LED systems, including embedded or fibre-optic illumination, have been proposed to address these limitations, but their implementation requires substantial infrastructural modification and cost investment.

From an economic standpoint, hatcheries must balance the potential improvements in chick quality, welfare, and later performance against the initial cost of retrofitting incubation equipment with programmable LED modules and the risk that suboptimal parameterization may reduce productivity. The benefits reported in research settings, such as enhanced muscle development, improved stress resilience, or superior feed efficiency, must therefore be validated through large-scale, multi-flock trials to confirm their financial feasibility under commercial conditions.

Despite these constraints, the increasing availability of energy-efficient LED technology and the growing emphasis on precision incubation have stimulated renewed interest in lighted incubation. As the industry progresses toward more welfare-oriented and data-driven production models, photostimulation may become an attractive tool for achieving predictable and favorable developmental outcomes. Collaborative research between academia, hatchery manufacturers, and integrators will be essential to establish species- and strain-tailored lighting guidelines, optimize integration with thermal and ventilation systems, and ensure consistent, reproducible benefits. Ultimately, addressing these challenges will be critical for the translation of experimental findings into scalable, commercially practical incubation strategies.

Light exposure during incubation acts as a potent biological cue that drives embryonic programming and shapes post-hatch success, modulating avian embryogenesis via distinct spectral pathways. While monochromatic green and blue wavelengths accelerate myogenic development, somatotropic signaling, and hatchability, red and white light profoundly influence melatonin dynamics, immune function, and stress responsiveness collectively enhancing structural development, behavioral phenotypes, and post-hatch adaptability. Despite these extensive biological benefits, widespread commercial adoption is restricted by unresolved optimal parameters for spectrum, photoperiod, and intensity across varying strains, species, and hatchery microclimates, alongside practical hurdles regarding energy costs, equipment engineering, and narrow thermal margins. The considerable variation in reported outcomes underscores the critical need to standardize experimental protocols to ensure reproducibility. Moving forward, the poultry industry must prioritize developing precision lighting systems aligned with temporal embryonic physiology, integrating photostimulation with broader incubation factors, and establishing an industry-aligned framework to generate evidence-based guidelines, ultimately leveraging lighted incubation to advance sustainable, welfare-oriented production.

All the authors of this manuscript earnestly acknowledge the support of their respective Universities and Institutes.

This review provides a comprehensive synthesis of recent advances in lighted incubation, bridging the gap between molecular mechanisms of avian photoreception (retinal and deep-brain pathways) and practical hatchery management. Unlike previous reviews, it critically evaluates how specific spectral wavelengths and photoperiods interact with physiological axes, embryonic metabolic programming, and post-hatch welfare, offering a roadmap for standardizing precision lighting protocols in commercial poultry production.

SA: Designed and supervised the article. SA, KE-S, GB, and BM: Data collection and writing the original draft. SA, KE-S, GB, BM, UE, AM and MA: Writing and editing the final version. All authors have given approval to the final version of the manuscript.

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During the preparation of this work, the author(s), in some sentences, used Google Gemini, Grammarly, and ChatGPT in order to improve the readability and language of the manuscript. After using this tool/service, the author(s) reviewed and edited the content as needed and take full responsibility for the content of the publication.

The authors have declared no conflict of interest.