Circadian rhythms in mice fed a single daily meal at different stages of lighting regimen.

Circadian rhythms in systemic and cellular variables were studied in three groups of mice on different schedules of daily food accessibility: (1) only during the first 4 hours of the 12-hour light span; (2) only during the first 4 hours of the 12-hour dark span; and (3) at all times. The amplitudes of circadian variation in rectal temperature, serum corticosterone, and liver glycogen were increased by "meal-feeding" in either early light or early darkness. The overall averages of corticosterone and glycogen were also increased by meal-feeding at either stage of the lighting regimen. The time of peak values in temperature, corticosterone, and glycogen were determined by the time of food presentation, regardless of its relation to the lighting regimen. On the other hand, the interval between food presentation and peak values in the corneal mitotic index was greater when feeding was restricted to early darkness. These differences among the three groups of animals resulted in different relations among varibles at any given interval after feeding onset. Such effects concerning total bodily function, energy storage, hormonal regulation, and basic cellular processes indicate the pertinence of meal timing to nutritional research and practice.