A comprehensive review published in LOHMANN INFORMATION compiles decades of research on the effect of lighting during incubation on hatchability and chick start. From cerebral lateralisation to feather pecking, through muscle growth and stress response: light before hatching matters more than we thought.
Introduction: a factor ignored for centuries
From the ancient incubators of Egypt and China to modern electric machines, the artificial incubation of eggs has been carried out in total darkness. Incubator manufacturers have perfected the control of temperature, humidity, turning and ventilation, but light was simply overlooked as a relevant factor for embryo development.
However, under natural conditions, the hen periodically leaves the nest, exposing the eggs to light-dark cycles. This biological reality has driven a growing line of research which, particularly since 2000, has generated significant evidence: chicken embryos perceive light, respond to it, and this exposure can have lasting consequences on their behaviour, health and productive performance.
This is documented in the extensive review published by Cissy Riedel and Barbara Tzschentke, researchers at Humboldt University of Berlin, in the journal LOHMANN INFORMATION. This article summarises their main findings and translates them into practical insights for the poultry professional.
When and how does the embryo perceive light?
One of the most striking findings of the review is how early the chicken embryo begins to detect light stimuli. The visual system develops progressively: the ocular blood vessels, the eyeball, the lens and the pineal gland form during the first days of incubation. The production of melatonin — the key hormone mediating the response to the light-dark cycle — begins around day 10 of incubation, and there is evidence that the pineal gland can respond to light stimuli as early as day 3.
Retinal photoreceptors differentiate later and respond to light stimulation from day 16 onwards, although activity has been detected as early as day 14. Full ocular functionality is reached between days 17 and 18. The chicken’s visual system is fully developed before hatch, on day 21.
A critical point highlighted by the authors: the eggshell modulates the spectrum and intensity of light reaching the embryo. Brown shells filter more light than white ones, which may explain why different genetic lines respond differently to the same lighting programmes during incubation.
Cerebral lateralisation: the brain “specialises” with light
Perhaps the most fascinating mechanism described in the review is cerebral lateralisation: the functional specialisation of the two brain hemispheres. Due to the embryo’s position inside the egg, only the right eye is oriented towards the shell and receives light stimulation. This triggers an asymmetry in brain development with profound behavioural consequences.
According to the reviewed literature, each hemisphere controls distinct functions: the left (connected to the right eye, which receives light) is responsible for learned and routine behaviour, such as discriminating objects and feeding. The right hemisphere (connected to the left eye) manages responses to danger, social recognition and hierarchy.
In embryos incubated in total darkness, this functional asymmetry does not develop adequately. The consequences are significant: chicks hatched in darkness show reduced ability to distinguish objects, unstable social hierarchies, lower learning capacity and greater fear behaviour. The last three days of incubation appear to be especially critical for this process.
Effects on embryonic development and hatch
Light accelerates development, but does not increase hatch weight
The review consistently documents that light exposure during incubation accelerates embryonic growth, regardless of the wavelength used. However, this does not translate into heavier chicks at hatch, as the embryo simply reaches the optimal hatch weight sooner, thereby shortening the incubation period. In some studies, continuous light reduced the time to hatch by up to 13 hours compared to total darkness.
Hatchability: no clear negative effect, but with nuances
There is no consistent effect of light on overall hatch rate. Photoperiods of 12 hours light and 12 hours darkness (12L:12D) tend to give slightly better results than total darkness or continuous light. A relevant finding is that the hatch window (the interval between the first and last chick to hatch) is reduced with photoperiodic lighting, which could improve flock uniformity.
A critical point: different genetic lines respond differently to the same lighting programmes. In studies with Barred Plymouth Rock, Lohmann Brown and Lohmann Lite, the response varied significantly among them, making it necessary to consider genotype when designing lighting protocols.
Light colour matters: the special case of green light
The availability of LED lighting has made it possible to investigate the effect of specific wavelengths. Of all the colours studied, monochromatic green light stands out for its effects on muscle growth. The review documents multiple studies showing that green light stimulates the proliferation of muscle satellite cells, mediated by an increase in levels of growth hormone (GH) and insulin-like growth factor (IGF). This effect was observed even with very brief exposure periods (15-minute light/15-minute dark cycles).
In broiler chicks, green light increased body weight and breast meat percentage up to 42 days of age. However, this effect depends on the genetic line: in Rhode Island Red layers, a weight increase was observed between 8 and 12 weeks, but not in White Leghorn, Columbia Rock or Barred Rock.
| Colour | Main documented effects | Observations |
| Green | Stimulates GH and IGF. Promotes muscle satellite cell proliferation. Shortens hatch time. Greater post-hatch weight in broilers. | Effect mediated by melatonin. Variable response depending on genetic line. The last days of incubation appear to be the critical period. |
| Red | Reduces fear indicators (TI, IT) when combined with white. Higher embryonic melatonin. Associated with more passive post-hatch behaviour. | The red + white combination improved hatchability in Leghorn, broilers and Pekin ducks. |
| Blue | Stimulates satellite cell proliferation. Greater post-hatch weight. Associated with more active behaviour. | Lower melatonin peak than red and white, which may explain the greater activity. |
| White | Reduces fear in behavioural tests. Lower stress (H/L ratio) under 12L:12D photoperiod. Greater initial weight. | Full-spectrum white LED is the most versatile and well-studied option. |
Intensity: more is not always better
The review cautions about an aspect that is often overlooked: light intensity can have detrimental effects if excessive. Intensities above 1,200–1,500 lux, even when applied for only a few hours per day, reduced hatchability in light brown and medium brown-shelled eggs, though not in dark brown-shelled eggs (whose pigmentation attenuates the intensity reaching the embryo).
The authors conclude that the positive effects of light are more likely when the intensity reaching the embryo is moderate, replicating the conditions of natural incubation in the relatively dim environment beneath the hen. An intensity of 150–250 lux with LED appears to be a reasonable range according to the majority of studies reviewed.
Post-hatch behavioural effects
Reduced fear and lower stress
Multiple reviewed studies agree that chicks incubated under lighting programmes show a reduced fear response in standard tests such as the tonic immobility test (TI), the isolation test and the emergence test. Chicks hatched in total darkness vocalised more during isolation and took longer to recover from tonic immobility, both classic indicators of greater fearfulness.
This effect is explained by the combination of two mechanisms: improved cerebral lateralisation (enabling better management of threat responses) and the establishment of a functional circadian rhythm (which reduces stress sensitivity). Indeed, the 12L:12D photoperiod proved superior to both total darkness and continuous light, as both extreme conditions prevent the development of a normal circadian rhythm.
Activity and feeding
The review finds no evidence that light during incubation increases overall activity levels in chicks. It was observed that chicks incubated under light showed greater feeding activity during the first two hours after lights-on, possibly as a result of a circadian rhythm already established during incubation. However, total feed intake was not affected, as feeding activity was compensated throughout the day.
Feather pecking: an indirect but promising relationship
The relationship between in ovo lighting and feather pecking is complex. An early study found greater gentle pecking in Leghorns incubated under light, but the authors caution that a very high intensity (250–1,000 lux) was used. At moderate intensities, other studies found no differences. Most interestingly, there is a possible indirect pathway: if light reduces fear and stress, and these are predisposing factors for severe feather pecking, then in ovo lighting could contribute to mitigating this problem. Some data with low-intensity green light suggest a tendency to reduce severe and aggressive pecking, with increased brain serotonin levels.
Skeletal health: continuous light can be detrimental
A relevant finding for management: continuous light (24L:0D) throughout incubation proved detrimental to leg bone development in broiler chicks, increasing the incidence of tibial dyschondroplasia during the grow-out phase. In contrast, the 12L:12D photoperiod was beneficial for long-term leg health. This reinforces the importance of not equating “more light” with “better outcomes”.
Six key conclusions of the review
| The authors summarise the state of knowledge in the following points: ✔ Light during incubation accelerates embryonic growth broadly, regardless of wavelength, but does not increase chick weight at hatch (the chick simply hatches earlier). ✔ There is no consistent effect on overall hatchability, chick quality or post-hatch performance in layers and broilers. ✔ High intensities are detrimental to hatchability, even with brief exposures. ✔ Green light stands out for its ability to stimulate post-hatch muscle growth via the GH and IGF pathways. ✔ Lighting programmes can improve the development of muscles, bones and bodily functions, facilitating adaptation to post-hatch conditions, especially when applied during the last days of incubation. ✔ No direct effects on feather pecking have been demonstrated, but the reduction of fear and stress could indirectly contribute to mitigating this problem. |
Practical implications for the industry
Recommendations for hatcheries
- 12L:12D photoperiod over continuous light or total darkness. The accumulated evidence favours photoperiodic cycles over extreme conditions. The 12L:12D regime promotes cerebral lateralisation, establishes circadian rhythms and reduces fear without the negative effects of continuous light on skeletal health.
- Moderate intensity (150–250 lux with LED). Avoid intensities above 1,000 lux, particularly with light-coloured or white-shelled eggs.
- Green LED for broilers. Where the objective is to enhance muscle growth, monochromatic green light has the strongest evidence base, even with brief exposures during the last days of incubation.
- Consider the genetic line. The response to light varies between breeds and between eggs of different shell colours. Protocols are not universal.
- Mandatory thermal control. LED technology minimises heat output, but any conventional light source can alter the thermal profile of the incubator. This must be strictly monitored.
Future outlook
The authors emphasise that the last days of incubation represent a “critical period” during which bodily functions are programmed for the bird’s entire life. Applying the “right” light during this period — correct in colour, intensity and duration — could stimulate embryonic development and adaptability to the post-hatch environment. They also suggest that the ideal lighting should be tailored to the specific characteristics of the bird’s eye (which differs substantially from the human eye), including an appropriate UV component. The risk of failing to do so is the development of persistent false chromatic vision that generates stress throughout the bird’s life.
They also note that knowledge derived from the natural incubation of domestic and wild hens could significantly guide the design of lighting protocols that replicate optimal biological conditions.
Source:
-. The effect of light during incubation on embryonic development and post-hatch behaviour, health and performance of domestic chicken. Cissy Riedel1 and Barbara Tzschentke1,2. 1Albrecht Daniel Thaer-Institute of Agricultural and Horticultural Sciences (ADTI) at Humboldt-Universität zu Berlin (HU). Lohmann Information 2024
Further reading:
-. Effects of LED lighting during incubation on layer and broiler hatchability, chick quality, stress susceptibility and post-hatch growth. Jesse C. Huth, Gregory S. Archer. Poultry Science, Volume 94, Issue 12, 2015, Pages 3052-3058, ISSN 0032-5791, https://doi.org/10.3382/ps/pev298.
-. In ovo lighting in poultry production
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