Figure 1. Phase delays or advances after copulation reduce the proportion of pregnancies carried to term.

(A) After copulation was verified via identification of vaginal plugs, mice were randomized into control (n = 12), phase-delay (n = 18) or phase-advance groups (n = 18). Zero to four days after copulation, mice were transferred to new light-tight cabinets, each with 12-hr light and 12-hr dark light cycles, but differing in the time of light onset and offset. Control females remained in this chamber on a constant 12∶12 light (yellow bar):dark (black bar) cycle that matched the preceding one during mating, whereas females in the experimental groups were exposed to either 6-hour delays or advances in the light cycle, which was repeated by switching cabinets every 5–6 days for the duration of gestation. (B) The number of copulations successfully carried to term in each of the groups was recorded via daily visual inspection. Data comparisons were made using Pearson’s chi-square test (Phase delays: χ² = 3.41, P<0.1; * Phase advances: χ² = 9.47, P<0.01).