(a) The Effect of Light or Dark Phase Feeding on Body Weight. Body weight (mean ± standard error) of B6 mice fed 60% high fat only during the 12 hour light phase (dashed line) or only during the 12 hour dark phase (solid line). Body weights were taken at the end of the 12 hour feeding phase in all animals. Similar significant differences were also observed when weights were taken at the end of the 12 hour fasting phase (data not shown). Within two weeks of maintenance on the high-fat diet, the light-fed animals weighed significantly more than the dark-fed animals (* = p < 0.05 light vs. dark-fed) and remained significantly heavier over the next 4 weeks (F (1,20) = 10.78, p < 0.004). (b) Weekly Activity Counts and Caloric Intake. Total weekly activity counts (squares) and calories (Kcal, triangles) are depicted for both light fed (dashed line) and dark fed (solid line) groups. Note that while over the 6 week period neither activity nor caloric intake differed significantly (p>0.10), the light fed group appears to be consuming more calories and moving less than the dark fed group. This raised the possibility that the additive effect of a small increase in caloric intake and a small decrease in activity can together contribute to specific differences in body weight.