Let sleeping rats lie: does the timing of husbandry procedures compromise laboratory rat welfare?

Research has indicated that chronic stress can reduce sleep quality and quantity. Yet there has been little investigation into whether husbandry procedures carried out during an animal's normal sleeping period affect subsequent sleep behaviour and welfare. We housed 48 male Wistar rats in enriched cages containing four rats, in either a light phase treatment (LP) (N=6 cages) or a dark phase treatment (DP) (N=6 cages) for five weeks on a 12h:12h light cycle with lights on at 0600. Rats in the LP cages were exposed to husbandry procedures (e.g. weighing, cleaning) 3.5h into the light phase when we would expect them to be inactive/sleeping, three times every week. DP rats experienced the same procedures, but 3.5h into the dark phase when we would expect them to be active/awake. After five weeks, behaviour for all cages was sampled over 12 hrs of both the light and dark phase. General health measures (e.g. body weight, weight gain and chromodacryorrhoea) were collected over the five weeks housing period, and organ weights (e.g. adrenal, thymus, spleen and testes) were recorded post-mortem. Principal component analyses yielded four factors each explaining over 10% of the variance. Two factors revealed treatment differences. For one, loading positively on chromodacryorrhoea score and negatively on sleep, self-grooming and enrichment directed behaviour, LP rats scored higher than DP rats (t10= 3.47, P<0.01). The other loaded positively for relative thymus weight and activity and negatively for aggression and social interaction. DP rats scored higher on this than LP rats (t10= -3.42, P<0.01). LP rats thus displayed indicators of reduced welfare (e.g. less sleep, elevated chromodacryorrhoea, lighter thymus glands, higher aggression) relative to DP rats. Therefore, husbandry procedures applied in the dark rather than the light phase might improve the welfare of laboratory rats