Nonphotic phase-shifting in clock mutant mice.

Nonphotic phase-shifting was studied in mice bearing the Clock mutation. First, free-running mice heterozygous for Clock and wild-type mice were induced to become active through a 4-h confinement to a novel running over 3 days. Second, mice exposed to light-dark cycle received daily hypocaloric food during 2 weeks, before being transferred to constant darkness and fed ad libitum. Behavioral activation during the mid-subjective day induced 40-min phase advances in the locomotor activity rhythm of wild-type mice, whereas it produced 50-min phase delays in the circadian behavior of Clock/+ mice. Calorie restriction phase-advanced by 80 min the locomotor activity rhythm in wild-type mice, but not in Clock/+ mice. Therefore, the response of the Clock/+ mice to nonphotic phase shifting differs from that of wild-type mice.