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When I was starting in the sleep field many years ago, the clinical wisdom was that napping was a bad idea. It was thought that taking a nap was especially unhelpful for people with insomnia. Basically, a nap was believed to decrease sleep drive and make it more difficult to sleep at night. Later research has shown that napping and its effect on night time sleep is more complex. Indeed, napping can improve alertness and can be used in such a way as to largely prevent problems falling asleep, even for those with insomnia. On the other hand, as napping has become more popular, some have advocated taking frequent, relatively brief naps as a way of increasing wake time and productivity while decreasing the need for night time sleep. The evidence for the effectiveness of this approach, however, is limited and risks causing sleep deprivation.
Several factors determine sleep drive. The first of these is the homeostatic drive that builds steadily across the course of the day. This drive is based on the length of wakefulness. The longer one is awake, the greater the drive to sleep becomes. For most people, after 16 hours of continuous wakefulness, mental functioning begins to decline significantly and sleepiness increases. As time goes on, staying awake becomes more difficult, and eventually it is virtually impossible to resist sleep. By taking a nap this drive can be satisfied earlier in the day, potentially leading to difficulty falling asleep at night.
A second factor that influences the propensity to sleep is the circadian rhythm. Our built-in 24-hour clock, located in the suprachiasmatic nuclei just above the optic chiasm, imposes a regular pattern of wakefulness and sleepiness over the course of the day. The circadian rhythm supports alertness in the morning, maintains wakefulness in the afternoon and early evening, and allows drowsiness as the sky grows darker.
The homeostatic drive influences the quality of sleep and is responsible primarily for the deep sleep that is experienced mostly in the first two sleep cycles of the night. This deep sleep is restorative and is associated with the timed release of hormones such as melatonin and human growth hormone. It is important for cell repair and recovery from the exertion of the day. The circadian rhythm, along with possible homeostatic factors related to the amount of deep sleep, influences rapid eye movement (REM) sleep. This “paradoxical sleep” involves a state that is difficult to be aroused from while at the same time showing EEG patterns suggestive of wakefulness. It occurs at the end of sleep cycles and in increasing long periods toward the morning.
A conflict can thus occur between deep and REM sleep if one’s bedtime is too late. In other words, the best time for deep sleep may have passed if a person goes to bed so late that the circadian rhythm is increasingly driving REM sleep. This same phenomenon can also occur for people with insomnia when an individual has trouble falling asleep and the early period of sleep occurs as the drive toward REM sleep is increasing.
Hopefully, this explanation makes clear why the impact of a nap is not static but changes across the course of the day. Since homeostatic drive is toward deep sleep and increases steadily as the day goes on, a nap will tend toward progressively deeper sleep the later in the day it occurs.
Sleep cycles follow an ultradian rhythm (a recurrent biological cycle that is less than 24 hours in duration) that is usually about 90 minutes in length. Sleep cycles typically start with light sleep (“stage 1” when, for example, a person is easily awakened by hearing their name called) to sleep that is of “average” depth (“stage 2”) to deep sleep (“stage 3” when it is very difficult to awaken a person). These stages are then followed by periods of REM sleep. As the night goes on the amount of time spent in deep sleep decreases while the time spent in REM increases. A nap in the morning will tend to have more of the lighter stages 1 and 2, with some REM possible, and as the homeostatic drive increases, the amount of deep sleep will also. Naps late in the evening will have more deep sleep. It is likely that everyone has had the experience of falling asleep near their bedtime while watching TV or reading, only to find that after awakening and going to bed for the night that it is very difficult to fall asleep.
It has become increasingly clear that taking a break for sleep in the middle of the day is a natural and indeed, helpful, part of the normal sleep/wake cycle. The circadian rhythm reaches a low point of alertness in the afternoon and then increases alertness until the early evening hours. The siesta is the most well know acknowledgment of the usefulness of an afternoon nap. It can help restore alertness and mental effectiveness without interfering with the ability to fall asleep at night.
How is this possible? First, the nap needs to be timed for the afternoon hours, generally around 2:00 PM. This is the famous “two o’clock drowsiness” that energy drinks, coffee, and brisk walks are supposed to help us get through. It is the natural low in the circadian rhythm that occurs before our internal clock begins to increase alertness to take us through to bedtime. A nap at this time is in synch with the circadian rhythm and will still be early enough in the day that not too much sleep drive has built up. This means that the depth of an afternoon nap will not be so deep as to significantly decrease the drive to sleep that night.
Second, the length of the nap needs to be shorter than a full 90-minute sleep cycle. If kept short, the nap will only be for a portion of a sleep cycle. If it terminates in stage 2 sleep, then it will be relatively easy to get up and get going and the nap will not significantly impact night time sleep. If, on the other hand, the nap goes on long enough that stage 3 deep sleep is achieved, it will be much more difficult to wake up and the napper may feel “groggy”, and worse than before the nap. This depth of sleep may also satisfy enough sleep drive that it will be harder to fall asleep at the regular bedtime. It’s something like having a double cheeseburger before going to a wonderful banquet. Just as a high fat snack ruins the appetite, so a deep nap will affect the regular night time sleep.
Now, sometimes we need to take naps even if they are going to negatively affect later sleep. The best example of this is when a traveler becomes drowsy while driving at night. In this situation, getting off the road and taking a brief nap can increase alertness and help decrease the danger of falling asleep at the wheel, although there may be greater difficulty sleeping after arriving at the hotel.
If you are suffering from insomnia you may benefit from cognitive behavioral techniques such as sleep restriction, which decreases time in bed in order to increase consolidation and depth of sleep. This technique can help people who have difficulty falling asleep or who have prolonged periods of wakefulness in the night. For some people with insomnia, there is an initial increase in day time sleepiness when starting sleep restriction. A short, well timed, nap may make it easier to implement the sleep restriction technique by helping restore day time alertness and improve functioning while the sleep pattern is changing.
Naps can help at those times when we’ve gotten less sleep than we needed the night before. A brief nap can help restore cognitive abilities without a significantly negative effect on night time sleep. Shift workers often benefit from naps to help compensate for the sleep loss experienced from working late or overnight shifts. In general, a brief afternoon nap can be a wonderful way to refresh for the rest of the day without depending on stimulating energy drinks or late afternoon coffee, which can further disrupt your night-time sleep. So work with your circadian rhythm and go for the siesta- nature’s way to a more alert and productive day!
Epstein, L. J. & Mardon, S. (2007). The Harvard Medical School Guide to A Good Night’s Sleep. New York: McGraw Hill.
