Effects of scheduled food and water access on circadian rhythms of hamsters in constant light, dark, and light:dark.

Wheel-running activity of 31 Syrian hamsters was monitored in constant light (LL), dark (DD), and light-dark (LD) under ad lib and restricted food and water access schedules. Free-running or damped circadian wheel-running rhythms of 26 hamsters in LL were entrained by exposure to the restriction schedule. The phase of entrainment was positive, i.e., wheel running anticipated access time. In LD, activity was nocturnal. When food and water were restricted to midlight, five hamsters showed additional activity in anticipation of access time. Thirty hamsters showed a small advance of the phase of nocturnal activity onset. In subsequent DD, anticipatory activity was revealed (unmasked) in several additional cases, and nocturnal activity shifted and entrained to the access schedule in 19 cases. The properties of food anticipation and entrainment under the three lighting conditions support a model of the hamster circadian system that incorporates separate but strongly coupled food/water- and light-entrainable pacemakers.