The phasing of circadian rhythms in mice kept under normal or short photoperiods

Under short-day conditions (L/D = 6:18 h), the evening onset of the main maximum of motor activity and food intake was delayed in relation to lights-off and the onset of the secondary maximum in the morning was advanced in relation to lights-on. Similar phase changes were found with respect to the onset of glycogen synthesis and its breakdown. In the case of melatonin, the rise and the fall occurred earlier or later, respectively, and the phase differences with respect to lights-off and lights-on became smaller. If the mid-point between the evening and morning onsets of activity and food intake and between the rises and falls of glycogen and melatonin concentrations are considered, then the changes in all rhythmic functions were similar. Consequently, the phase relationships between different rhythmic functions and with respect to the light-dark cycle, and thus the internal and the external temporal order, were preserved under different light-dark ratios. Also, the results are in good accordance with a modified two-oscillator model where lights-off triggers advances of an evening oscillator and lights-on delays a morning oscillator.