Time course of sleep and NREMS EEG delta power during constant dark conditions (DD). Layout and symbols are same as in Figure 1. Gray bars at the bottom of each panel mark intervals with significant genotype differences (P < 0.05, post-hoc t-tests). The subjective day is marked with a gray horizontal bar at the top of the upper panel. The first 12-h represents the last dark-period under baseline (BSL) light-dark conditions.