Light is an important environmental factor in poultry production and acts as a strong developmental modulator during the incubation of chicken eggs, influencing embryonic growth, physiology, behaviour and subsequent chick performance. Photoperiods of 12L:12D or 16L:8D generally accelerate development, improve hatch synchrony and welfare indicators, while constant light or constant darkness increase the risk of hormonal imbalance. The appropriate intensity depends on the context: low to medium levels (5–50 lx) are optimal for broilers, while laying hens require higher levels (≥80–100 lx). Spectrally, red light is associated with a shorter incubation period and favourable immune responses; green light promotes skeletal muscle growth and can shorten the time to hatching; blue light reduces aggression and supports antioxidant mechanisms. LED light sources are favoured over fluorescent and incandescent lamps due to their low heat output, energy efficiency and precise spectral control. Light acts through the retina, pineal gland and photoreceptors in the deep brain by modulating melatonin, GH, GnRH and corticosterone, leading to greater vitality, improved navel closure, immunity and learning. Critical windows of sensitivity include the early (ED1–ED3) and late stages (ED14– ED18) when vision and circadian rhythms mature. The characteristics of the eggshells have a strong influence on the spectrum: brown, more pigmented shells absorb UV light and more short-wave light, while white shells transmit more blue-green light; together with breed differences, this explains the heterogeneous responses. An optimal lighting programme should therefore be tailored to the eggshell, breed and production goals in terms of spectrum, intensity and timing.
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