1 p.m. to 2:30 p.m.

Doctors used to think afternoon sleepiness was the result of a big lunch. "But we've found that in the early afternoon there�s a dip in body temperature, which causes sleepiness," says Michael Smolensky, a professor of environmental physiology at the University of Texas School of Public Health at Houston and author of The Body Clock Guide to Better Health (Owl Books, $11, www.amazon.com). Just as a similar decrease encourages you to shut down at bedtime, this midday dip can make you crave a siesta. An ideal nap, he says, should last 15 to 20 minutes. More than 30 and you may end up with sleep inertia — and feel even more groggy when the nap's over. Richard Schwab, M.D., codirector of the University of Pennsylvania Penn Sleep Center, in Philadelphia, says that early afternoon is indeed when your circadian rhythms (the pattern of physical and mental changes we each repeat every 24 hours) are "more likely to want your body to sleep." But Schwab insists that if we weren't all so sleep-deprived, we wouldn't need naps in the first place.