Photic entrainment in hamsters: effects of simulated twilights and nest box availability. J Biol Rhythms

Abstract Entrainment of wheel-running activity rhythms was compared in ham-sters exposed to daily light-dark (LD) cycles with abrupt transitions between 0 and 10 lux or with artificial twilights simulating summer solstice conditions at 41°N latitude but truncated at 10 lux. The photoperiod in LD-rectangular was set at 16.24 h, equating the total light (in lux • min) emitted under the two schedules. The LD cycles were maintained for 35 days and were followed by 14 days of constant darkness (DD). Half the animals in each condition had access to a dark nest box connected to the outer compartment by a tunnel, the remaining animals being confined to a single compartment. Body temperature and locomotor activity inside the nest boxes were recorded by telemetry. Movements between the nest box and the outer compartment were monitored and the data were used to calculate light exposure at different times of the day. In all groups, the phase angle difference between wheel-running onset and dusk was more positive than that between activity offset and dawn. Hamsters with access to nest boxes, however, had later onsets, earlier offsets, and shorter activity durations (αs) than thos