Red light at night permits the nocturnal rise of melatonin production in horses

Abstract Exposure to white light at night suppresses melatonin production, impacts circadian rhythms and contributes to ill-health in humans. Human interaction with horses frequently occurs at night. We tested the hypothesis that dim red light would not suppress the nightly rise in serum melatonin levels in horses. In a crossover design, six horses were maintained for consecutive 48 h periods under a Light:Red (LR) and a Light:Dark (LD) photo-schedule. Transitions from light (>200 lux, polychromatic white light) to red (5 lux, peak wavelength 625 nm) or dark ( No time x treatment interaction or effect of treatment on serum melatonin levels were demonstrated (P > 0.05). A robust main effect of time (P