The decrease in delta power in the first 6 hr after the 6 hr SD followed an exponential decline. The parameters defining this function are the time constant (τ_d), the level of S reached at the end of the SD (S₀), and the lower asymptote (LA), which were determined by non-linear fitting (see Materials and Methods). Only S₀ varied with genotype [mean (SEM); p values indicate the probability that the parameter does not vary with genotype; one-way ANOVA factor strain; S₀: AK = Br = C = B6 > Br = C = B6 = 129 = D2; p < 0.05; Tukey's range test; n = 7 per strain]. Notice that here S₀ indicates the value of S at the time of recovery sleep onset, whereas elsewhere the level of S at light onset of baseline is meant.