Influence of lighting on sleep behaviour, circadian rhythm and spontaneous blink rates in stabled riding school horses (Equus caballus)

Application: The study supports the use of a red/white light system to standardize the light/dark cycle without negatively influencing sleep behavior or circadian synchrony of stabled horses. It also supports the importance of distinct and routine light/dark periods to promote wellbeing of stabled horses.

Introduction: Modern horse husbandry can involve significant time spent indoors, often in suboptimal lighting conditions and with frequent night-time disturbances by humans for management purposes (Lesimple et al. 2016). The aim of this study was to investigate the influence of a customised light-emitting diode (LED) lighting system and a standard fluorescent lighting fixture on equine sleep behaviours, circadian rhythmicity, and spontaneous blink rates in horses.

Materials and Methods: Ten riding school horses experienced two stable lighting conditions for four weeks each in a cross-over study running from January to March 2023 (Figure 1) at an Equestrian Centre in Gloucestershire, UK. The treatment lighting consisted of an LED system that provided timed, blue-enriched white polychromatic light by day (peak wavelength 460 nm and mean light intensity 487.8±41.5 lux) and dim red light (peak wavelength 620 nm and mean light intensity 9.7±0.2 lux) at night. Lights transitioned gradually over 20 min from red to white at dawn (07:20) and white to red at dusk (20:20) to give approximately 13 h of daytime light when in use. Control lighting was a fluorescent tube (peak wavelength 560nm and mean light intensity of 227.8±7.5 lux) that was turned on and off manually morning and evening. During week 4 of each experimental period, spontaneous blink rate was recorded twice for 30 min by securely attaching Go-Pro Hero 10 cameras (7 x 5 x 4 cm Go-Pro Incorporated) to the headcollars of each horse individually. Analysis via Shapiro-Wilk’s test indicated that the data sets were normally distributed and Paired t-tests were used to compare mean values of full blinks under both lighting systems. Behaviour of horses in their stables was recorded using Hikivision CCTV equipment comprising a 16 channel 4K Power over Ethernet (PoE) Network Video Recorder (NVR) against a pre-defined ethogram (Greening et al. 2021) and focal continuous sampling for 72 consecutive hours. Statistical differences between lighting conditions (Treatment versus Control) were assessed using paired t-tests or Wilcoxon tests where data were shown to be normally or not normally distributed, respectively. One-way repeated measures (RM) ANOVA or a Friedman’s test was conducted to determine the impact of the lighting period (Day or Night) on the horses' behaviour depending on data normality. Mane hair samples complete with follicles were collected at 4-h intervals for 52 consecutive hours, following which samples were analysed in the lab to identify expression of circadian clock genes. RM two-way ANOVA was used to assess the effect of time, treatment, and time x treatment interaction on gene expression in all horses. All data were analysed using GraphPad Prism version 10.3.1 for Windows, presented as means (± S.E.M.) and considered statistically significant if P<0.05.

Results: No differences were detected for total sleep, lateral or sternal recumbency, wakefulness, standing, standing sleep, or spontaneous blink rate (P>0.05), between lighting conditions (Table 1). The lighting period (Day versus Night) had an effect on total sleep (P<0.01) (Figure 1), total recumbency (P<0.01), wakefulness (P<0.01), and standing sleep (P<0.05) in both conditions. For the treatment condition only, higher wakefulness was recorded during Day (P<0.05). An overall effect of time for clock genes PER2 and DBP was detected (P<0.01), but there was no effect of treatment, or time by treatment interaction. Cosinor analysis detected significant 24-h rhythmicity for PER2 and DBP (P<0.01) in both lighting conditions.

Conclusions: Results imply that dim red light at night does not negatively impact levels of arousal, normal sleep patterns, or circadian rhythmicity. Blue-enriched LED light may promote increased wakefulness during daytime in stabled horses. These results contribute to our understanding of how stable lighting can promote healthy sleep behaviour patterns and optimise circadian health in horses and provide a foundation to inform future research and practice in this important area.

References:
Greening, L., Downing, J., Amiouny, D., Lekang, L. and McBride, S. (2021) The effect of altering routine husbandry factors on sleep duration and memory consolidation in the horse. Applied Animal Behaviour Science. 236, 105229.
Lesimple, C., Poissonnet, A., and Hausberger, M. (2016) How to keep your horse safe? An epidemiological study about management practices. Applied Animal Behaviour Science. 181, 105-114.