Humans in their adult years compared to most other species don’t usually nap.
Napping is part of the sleep cycle of infants and children, and is quite common in the elderly population. There is a natural and biological propensity to nap during the day. This is due to a dip in the circadian rhythm of the body – the 24 hour sleep wake cycle that is determined both by the daylight cycle, and by the body’s internal 24 hour cycle. (By the way circadian (from the latin “circa” and “dies”, meaning: “about 24 hours”).
The 2 sleepiest points in the body’s 24 hour cycle are between 2 am and 4 am (the worst time to have to wake up), and between 1 pm and 3 pm. This afternoon dip in the cycle is due to the fact that the waking effect of daylight hasn’t quite kicked in fully, and the sleep pressure (the urge to sleep that accumulates from being awake) is starting to accumulate and make us sleepy. Add to that the effect of lunch, and this explains why a nap around 2pm might be really desirable.
Of course some cultures have been napping in the afternoon for these and other reasons. In many Mediterranean countries and especially in Spain, land of the siesta, the scorching summer afternoon sun is another good reason to take a nap, a long nap.