Abstract
In birds, light can penetrate the cranial bones and reach deep brain regions, where non-visual photoreceptors, especially in the hypothalamus, detect spectral and photoperiodic cues. Alongside retinal photoreception, deep-brain light sensing contributes to circadian entrainment and regulates melatonin secretion by the pineal gland. These light-driven pathways modulate endocrine activity, playing a key role in muscle development. This review explores how monochromatic light-emitting diode (LED) illumination, particularly green and blue wavelengths, affects the somatotropic axis (growth hormone-releasing hormone [GHRH]-growth hormone [GH]-insulin-like growth factor 1 [IGF-1]), the gonadal axis (gonadotropin-releasing hormone [GnRH]-luteinizing hormone [LH]/follicle-stimulating hormone [FSH]-sex steroids [testosterone, estrogen, progesterone]), the thyroid axis (thyrotropin-releasing hormone [TRH]-thyroid-stimulating hormone [TSH]-thyroxine [T(4)]/triiodothyronine [T(3)]), and the hypothalamic-pituitary-adrenal (HPA) axis (corticotropin-releasing hormone [CRH]-adrenocorticotropic hormone [ACTH]-corticosterone). Green light enhances early-stage muscle growth via GHRH and IGF-1 upregulation, while blue light supports later myogenic activity and oxidative balance. Light schedules also influence melatonin dynamics, which in turn modulate endocrine axis responsiveness to photic cues. Furthermore, variations in photoperiod and exposure to artificial lights at night (ALAN) affect thyroid activity and HPA axis reactivity, influencing metabolism, thermoregulation, and stress resilience. Together, ocular and intracranial photoreception form a complex network that links environmental light to hormonal regulation and muscle growth. These insights support the strategic use of LED lighting to optimize broiler performance and welfare.