Red light at night permits the nocturnal rise of melatonin production in horses. (October 2019)

• Record Type: Journal Article
• Authors: Murphy, B.A.; O'Brien, C.; Elliott, J.A.
• Is Part Of: Veterinary journal. Volume 252(2019)
• Publication Date: 2019-10
• Subjects: Circadian rhythm -- Equine -- Light at night -- Melatonin; Veterinary medicine -- Periodicals; 636
• DOI: 10.1016/j.tvjl.2019.105360
• Languages: English
• ISSNs: 1090-0233

Abstract

Graphical abstract: Highlights: Low intensity red light at night permits the nocturnal rise in serum melatonin in horses. Similar 24-h melatonin production patterns were observed in horses maintained under darkness or red light at night. Red light at night permits visibility and is an alternative to white light to avoid circadian rhythm disruption in horses. Nocturnal red light use in equine stabling may provide important health benefits for 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 (<0.5 lux), and vice versa, coincided with ambient sunset and sunrise times. Blood was collected at 2 h intervals for 24 h during each treatment via indwelling jugular catheters. Samples were harvested for serum and stored at −20 °C until assayed for melatonin by radioimmunoassay. Repeated measures two-way ANOVA and t -tests analysed for differences in LR and LD melatonin profiles and their circadian rhythm parameters. No time × treatment interaction or effect of treatment on serum melatonin levels were demonstrated ( P > 0.05). A robust main effect of time (P < 0.0001 ) predominated, with melatonin levels rising at night under both treatments. Statistically significant differences were not observed when LR and LD were compared for circadian rhythm measures of night time peak, area under the curve (AUC), or for times of onset (evening rise), offset (morning decline), or peak duration. Low intensity red light at night did not impact the pattern of melatonin secretion in this study and is, therefore, unlikely to impact the physiology of circadian or seasonal regulation.