Physiological and cognitive consequences of a daily 26h photoperiod in a primate (M. murinus)

Daily resetting of the circadian clock to the 24h natural photoperiod might induce marginal costs that would accumulate over time and forward affect fitness. It was proposed as the circadian resonance theory by Pittendrigh in 1972. For the first time, we aimed to evaluate these physiological and cognitive costs that would partially explain the mechanisms of the circadian resonance hypothesis. We evaluated the potential costs of imposing a 26h photoperiodic regimen compared to the classical 24h entrainment measuring several physiological and cognitive parameters (body temperature, energetic expenditure, oxidative stress, cognitive performances). We found significant higher resting body temperature and energy expenditure and lower cognitive performances when the photoperiodic cycle length was 26h. Together these results suggest that a great deviation of external cycles from 24h leads to daily greater synchronization costs, and lower cognitive capacities. To our knowledge, this study is the first to highlight potential mechanisms of circadian resonance theory.