HOW DO WE EXPLAIN naps and siestas? The best explanation we have at this time involves the relationship between sleep pressure and circadian rhythms. Sleep pressure starts to build when we wake up. Assuming we wake up in the early- to mid-morning, it will reach a fairly high level by early afternoon, roughly halfway through the waking day.
Meanwhile, the morning burst of cortisol has worn off and the rise in core body temperature, another wakefulness signal, is only starting its gradual ascent toward a peak in the second half of the day. (It may even go through a small dip at this time of day.) The drive to sleep is strong enough to override the drive to stay awake and the result is that napping becomes a possibility.
Food can be an added pro-nap factor. Eating a large or heavy meal at lunchtime causes an insulin response that leads to a temporary drop in blood glucose. This in turn promotes drowsiness. However, the primary reason that siestas are both attractive and possible is clearly circadian. Even someone who skips lunch can end up wanting a nap because of accumulated sleep pressure combined with low levels of wakefulness signals.
Is taking a siesta a good idea? Quite a lot of people have thought so, including Napoleon Bonaparte, Thomas Edison, Albert Einstein, and Winston Churchill. And lab studies back them up. After a nap, people are more alert and productive, less tired, and more positive in their mood. Their logical reasoning and decision-making skills improve, and if they have been learning something new or practicing a new skill, they retain it better after a nap. There is even research suggesting that regular napping reduces the risk of heart attack, stroke, diabetes, and obesity. What’s not to like?
Still, some cautions are in order. It matters when we nap and how long we nap. Studies in which people were encouraged to take naps at different times of day confirm what most of us probably already suspect. For adults with a normal sleep-wake pattern, the best time to nap is in the early afternoon, about halfway between morning wake-up and evening bedtime. Napping later in the day, however, may have an unfortunate impact that evening. Naps deplete sleep pressure, and the later that happens, the less time there is for sleep pressure to build up again by bedtime.
It is still making its way up toward the “ready-to sleep” level at bedtime, when your circadian rhythms of melatonin release and body temperature drop have already reached that point. Sleep pressure and the circadian cycle have gotten temporarily out of sync, and until they get back in harmony, you will have trouble falling asleep.
Researchers have also studied the effects of varying the length of naps. Surprisingly, it turns out that alertness and cognitive skills improve after as little as ten minutes of napping. That is enough time to relieve fatigue, too. Naps of twenty minutes or half an hour do not provide any greater benefits, and they are also more likely to set off a period of grogginess or sleep inertia. Those who take longer naps, of an hour or more, build up even more sleep inertia. There may be other gains from their deeper sleep, such as more creative problem solving, but it takes them even more time to return to full alertness and effectiveness.
Finally, a word to the wise: Don’t rely on napping to compensate for artificially short sleep during the workweek. Healthy sleep means adequate sleep every day, although inevitably exceptions will occur. Do your best to keep bedtime and rise time consistent ― and that includes weekends! By going to sleep just 30 minutes earlier than usual, you can get 3½ hours extra sleep evenly distributed across the week. That’s the same total sleep you would get with 30-minute naps every day, or by oversleeping 1¾ hours on Saturday and Sunday. The napping strategy is preferable to the oversleeping strategy, however, since oversleeping allows your circadian clock to drift later, making healthy bedtimes and rise times even more difficult to achieve starting Monday.
This post is adapted in part from my paperback, Reset Your Inner Clock, which features timed light therapy as a solution for circadian insomnia ― the chronic inability to get to sleep on time, which can kill vigorous activity and top concentration during the day.