Topography of homeostatic sleep pressure dissipation across the night in young and middle-aged men and women.

Decline in slow-wave activity (SWA) across the night is believed to reflect dissipation of the homeostatic sleep drive. This study evaluated the effects of age, sex and topography on SWA dissipation. The sleep electroencephalogram of 48 young [22 women, 26 men; mean = 23.3 years; standard deviation (SD) = 2.4] and 39 middle-aged (21 women, 18 men; mean = 51.9 years; SD = 4.6) healthy volunteers was analysed. Spectral analysis (0.5-22.0 Hz) was performed per non-rapid eye movement period for Fp1, F3, C3, P3 and O1. SWA (1.0-5.0 Hz) dissipation was modelled using linear and exponential decay functions applied to each age and sex subgroup data set for each derivation. The relative adequacy of both functions was compared using Akaike's information criterion. Results suggest that the exponential model provides a better data fit than the linear fit independently of age, gender and brain location. In women, age reduced the span (distance between the y intercept and the asymptote) of SWA decay in Fp1, F3, P3 and O1. In men, however, the effect of age on the span of SWA decay was limited to Fp1 and F3. In all age and sex subgroups, anterior regions showed a higher span than posterior regions. The asymptote was lower in anterior regions in young but not in middle-aged subjects. These results suggest that the homeostatic process operates on a larger scale in anterior regions. Importantly, ageing reduced the scale of homeostatic dissipation in both sexes, but this effect was more widespread across the brain in women.