Kimberly Cote, director of the Sleep Research Labora-tory at Brock University in Ontario, answers:

Daytime napping in healthy adults does indeed lead to benefits in terms of alertness, mood and cognitive functioning. Adults do not require shut-eye in the middle of the day—unlike infants and toddlers—but many grown-ups nap just the same. A 2008 National Sleep Foundation poll found that 460 out of 1,000 respondents had napped at least twice during the previous month.

People cite a variety of reasons for indulging in daytime siestas. Some take so-called replacement naps to make up for poor sleep the night before. Shift workers may take prophylactic naps in anticipation of needing to stay awake overnight. Many others, regardless of age and culture, habitually take appetitive naps—they sleep simply because it feels good.

Intuitively most of us think that a nap will refresh us and make us better able to take on the challenges of the day. In fact, research shows that healthy adults who take naps enjoy brighter moods, faster reaction times, and better performance on tasks involving logical reasoning, attention and memory.

How much we gain from napping, though, depends on a number of factors, including how and when we nap and for how long. A 20-minute nap appears to hit the sweet spot. Studies reveal that such brief sojourns boost both mood and cognitive performance. Shorter, 10-minute naps are also good for enhancing performance and cause less grogginess than longer naps do.

Naps lasting an hour or more are not recommended. During a longer nap, you fall into a deeper sleep, which makes it more difficult to awaken feeling refreshed. In other words, the longer the nap the greater the “hangover” effect afterward. Also, longer naps diminish the quality of nighttime sleep.

The best time of day to take a nap (assuming you keep a regular night sleep schedule) is midafternoon, between 2 and 4 P.M. Given the body's natural biological clock, it is generally easier to fall asleep during this window and to reap the full benefits of a good rest.

In one study from our sleep laboratory, we found that habitual nappers slept more lightly than nonhabitual nappers did, which may mean that the ability to nap lightly contributes to better alertness and performance after napping. Habitual nappers also reported feeling better than the nonhabitual nappers after the same amount of sleep.

Though generally beneficial, napping isn't for everyone. Poor sleepers who have difficulty falling and staying asleep at night might want to avoid daytime snoozing. For everyone else, though, a 20-minute midafternoon nap could be the secret to feeling sharp and happy throughout the day.