NIOSH Training for Nurses on Shift Work and Long Work Hours
Afternoon Dip in Wakefulness
Figure 2.7 explains the underlying processes of why humans tend to feel sleepy and take a nap in the middle of the afternoon. At this time, circadian rhythms that promote wakefulness dip, and sleep drive builds up enough so that pressure to sleep dominates.
Also note that in the evening, the circadian rhythms for wakefulness dominate over the buildup of sleep drive that has occurred during the long period of being awake. This boost in wakefulness can make it difficult to fall asleep early.