Our understanding of the mechanisms by which sleep deteriorates with age almost exclusively stems from comparisons of young and elderly subjects. The present study investigated the different effects of a 25-h sleep deprivation on the recovery sleep initiated in the morning (when circadian sleep propensity decreases) of young (20-39 y) and middle-aged subjects (40-60 y). Middle-aged subjects showed a steeper increase in the duration of wakefulness during daytime recovery sleep than the young subjects. Slow-wave sleep (SWS) and EEG slow-wave activity (SWA: spectral power between 0.5-4.5 Hz) were potentiated in both groups following sleep deprivation. However, the rebound of SWS and SWA was significantly less pronounced in the middle-aged than in the young. This reduction in homeostatic recuperative drive in middle-aged subjects might account for the decrease in their ability to maintain sleep when they have to recuperate at an abnormal circadian phase. These results helps to understand the increase in complaints related to shift work and jet lag in the middle years of life.