Loss of circadian behavioural rhythms in rats kept in constant darkness

Abstract Ten male Sprague–Dawley rats were exposed to 1-h light and 1-h dark (LD 1 : 1) cycles for 50 days. They were then released into constant darkness (DD) for 104 days. Exposure to LD 1 : 1 caused gradual disruption of circadian rhythms in their ambulatory and drinking activities until, finally, all the animals lost their circadian behavioural rhythms. After their release into DD, eight rats showed free-running circadian behavioural rhythms, whereas the remaining two rats showed circadian arrhythmicity for approximately 50 days in DD before they restored their free-running rhythms spontaneously.