AVP-expressing cells per section (mean ± SEM) over time in young (•), middle-aged (○), and old females (▾). The two-way ANOVA examining the effects of age and time of day on AVP cell number revealed a significant effect of time (F(6,108) = 11.19; p < 0.001), with the number of AVP-expressing cells gradually increasing between 0300 and 1600 hr and then declining to baseline levels by 2300 hr (different letters are significantly different from each other; p < 0.05). The black bar under the x-axis represents the dark phase of the cycle, and the white bar represents the light phase.