Best time for napping

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This text is part of: "Science of sleep" by Piotr Wozniak (2017)

Value of siesta

Naps are a blessing for a tired brain. However, if taken at a wrong time, they can also contribute to messing up your sleep cycle. Many people believe that a nap is a nap is a nap. Whatever its timing, the nap will refresh your mind. This is false. Understanding the optimum circadian timing of naps is essential for naps to be your friend, not your enemy! The belief in the universality of naps sparked a dangerous ideavirus: lifestyle based on polyphasic sleep.

Napping is a skill. Many people cannot nap even if they are sleepy. Measuring the time between your natural waking and the nap should help you optimize the quality of a nap. Optimally, your tiredness might not even be perceptible enough to easily guess the optimum timing. If you measure the time between night sleep and the nap, you will notice that the length is always the same (minor variations depend on the quality of sleep in the night). In other words, the measurement helps you figure out the timing of your circadian dip even on days when you do not feel tired at midday. You may wonder, why nap in the first place then? The boost in cognitive powers is worth the investment (which may be as little as 10-20 min. on a good day).

In a healthy biphasic sleep, a nap taken at siesta time is an excellent boost to your mental energy and creative powers. It is important to know that the timing of the nap should not be determined by the clock that hangs on your wall. Your nap should come at around 7-8 hour of your natural waking time. To be precise, only you can determine that value precisely by comparing what happens if you try to take naps a bit earlier or a bit later. The optimum value may not hold if you cut your sleep short with an alarm clock, or fall asleep earlier than usual (e.g. because of an exhausting day), or delay going to sleep beyond your natural sleep hour.

Circadian nap phases

To optimize your nap taking, you need to understand the impact of the circadian phase on sleepiness. Below, I explain what happens if you take naps at different phases. In the text below, Phase 7 Nap denotes a nap that is taken 7 hours after natural awakening from sleep taken at natural hours. Refer to the following graph which illustrates the biphasic nature of human sleep propensity. This graph shows that there is only one optimum alertness valley (in red) conductive to sleep, usually in hours 7-8:

Grades of a biphasic sleeper in relation to the circadian phase showing two major peaks in alertness and learning quality during the day

Figure: Recall at different times of day in a biphasic sleeper in relation to the circadian phase showing two major peaks in alertness and learning quality during the day. In monophasic sleepers, the same peaks can be observed, however, evening recall may be lower. This means that biphasic sleeper can expect higher overall learning or creativity in the evening. 58,339 SuperMemo repetitions were used to produce this graph. The alertness in purple color is determined by a sleep model based on the two-process model of sleep regulation. Alertness estimate combines the inverse of homeostatic and circadian sleep propensity

Phase 0: Waking time

Napping in Phase 0 is napping that takes place immediately after waking, i.e. circadian phase 0 in the graph above. Napping in Phase 0 is possible, and largely depends on the history of prior sleep. Phase 0 naps after a normal night sleep can be considered as a complement to the night sleep, e.g. if it was not effective enough. Such naps consolidate with the night sleep in sleep models and are an efficient way of extending the night sleep in cases when it was interrupted (e.g. by noises, bursting bladder, health issues, etc.). Phase 0 naps after a sleepless night can serve as an inefficient substitute for the night sleep. Such sleep will be short, unrefreshing and leave a sleep debt. It will also introduce unwelcome oscillations in the circadian system that may take a few days to clear up. Such sleep is often used by night-shift workers to get some mental boost for the day. Sleep in Phase 0 is still far better than no sleep at all. The rule is simple:

If you are sleepy in Phase 0, nap at will. Your brain clearly needs more sleep

Phase 3: Creativity time

Napping in Phase 3 should never be possible in a healthy well-regulated system (see the red peak in hour 3 on the horizontal axis in the graph above). Successful sleep at this time is an indication of sleep deprivation, poor quality sleep (e.g. due to sleep apnea), sleep in a wrong phase (e.g. taken too early), sleep disorder (e.g. narcolepsy), etc. This is probably the hardest time to nap of all. However, I am not aware of any bad effects of such naps for health or for the sleep control system.

Phase 5: Pre-siesta

Napping in pre-siesta slot is possible. However, such naps are likely to be short and not as refreshing as Phase 7 naps. They are also more likely to be REM-rich for circadian reasons. Those early naps can probably be recommended to people who suffer from sleep-onset insomnia, and who still want to boost the second half of their day in terms of alertness and creativity. Those naps can also be executed "in a hurry" due to their short duration in cases where longer napping is undesirable, or later timing does not fit the day's schedule.

Phase 7: Siesta

Perfect time for napping. As it can be seen in the graph, Phase 7 is the period when the mental performance is at its mid-day nadir (aim at Phase 7 to make sure being late will still place the nap within the nadir). It is not true that the nadir is caused by a hefty lunch (even though meals have a big impact on sleep control). The nadir is a natural expression of the circadian wave. This circadian phase low time comes at the roughly same clock time as the subjective night nadir at a roughly 12 hour shift (e.g. if the middle of your night falls at 3 am, naps at 3 pm could be most effective). This is well explained in "How to nap". The benefits of a siesta have been confirmed by numerous studies. It has been practised for ages in many regions of the world. It will definitely trickle into the corporate world as human productivity becomes increasingly dependent on our creative powers.

Naps taken 7 hours from natural waking are best for the brain, health and productivity

Phase 11: Evening

This is not a good time for napping. In a healthy cycle, napping might be hard to achieve or impossible. However, even a minor degree of sleep deprivation will produce a nap that might trigger the control mechanisms responsible for the full-night sleep. Late naps are likely to be rich in NREM sleep and rob your night sleep of the vital SWS component (Werth et al. 1996[1]). Those naps can last far longer than siesta naps. They can make you groggy. Worst of all, they can compound insomnia. Unfortunately, this is a type of a nap that a huge proportion of students take! Forced to wake up at indecently early times for school, kids and students struggle semi-conscious through school hours with negligible progress in learning. Learning in such a state only magnifies the pretty universal hatred of school. Phase 11 nap is then the only way to survive the day and get some actual learning done in the evening. The body clock shifts the subjective night to morning hours. The positive side effect is that evenings can be filled with effective studying. The negative side effect is that the student finds it impossible to fall asleep before 3-4 am, and welcomes the new bright school day with an alarm clock that rings in the middle of the subjective night. This perpetuates the cycle of suffering and school hate. Nobody has ever estimated the global consequences of this phenomenon that includes an impact on adolescent attitudes that are notoriously fraught with problems. Neither has anyone come up with a practical solution (shifting school hours usually results in kids "adapting" to the new cycle by shifting their bed time as well). I am not able to recommend a solution here either. Skipping evening naps might be better for the quality of night sleep and for the stabilization of the circadian cycle in the earlier phase, however, that would effectively rob those students of their only time in which they can learn. Those evening naps are also the only meager substitute for free running sleep that those young brains crave. The only time when the brain gets what it wants. If I was to answer: to nap or not to nap, I would probably have to admit that evening napping is the lesser evil in a majority of cases.

Phase 13: Pre-sleep

This is a particularly bad time for napping. Initiating naps at this time should be relatively easy. However, pre-sleep naps are likely to produce one of the following unwelcome outcomes: long-nap-short-night or long-night-early-waking (depending on the current status of the sleep control system). A pre-sleep nap is likely to result in triggering the night sleep sequence. However, this sequence is not unbreakable, and can result in early awakening combined with the difficulty in launching back to sleep (insomnia). This is particularly likely if the homeostatic sleep process generates substantial sleepiness while the circadian sleep process is not yet mature for the night sleep. As a result, such a pre-sleep nap can yield less total sleep than a normal night sleep. This long-nap-short-night will not entirely fulfill the physiological function of sleep. Consequently, your alertness levels for the next day are likely to dip substantially. The less unfortunate outcome of a pre-sleep nap is if you successfully trigger the uninterrupted night sleep sequence. However, you will likely prematurely run out of the homeostatic process before the circadian function of sleep is completed. You will probably wake up earlier than usual. This is the long-night-early-waking outcome that produces nights that are amazingly unrefreshing considering the fact that premature sleep is often much longer than an ordinary night sleep. The reason for this low sleep efficiency is probably the scarcity of REM sleep, which is strongly circadian. Moreover, for circadian reasons, your morning is likely to be unusually sleepy!

Phase 15: Segmented sleep

Phase 15 napping should be considered "early night sleep". If you go to sleep at this time you can expect any of the following (depending on the degree of sleep debt):

  • if you carry no sleep debt: some unproductive time in bed, as you might not be able to fall asleep.
  • pre-sleep outcome as in Phase 13 sleep: long nap with a short night, or long night with early waking. This outcome is likely to leave you less refreshed.
  • segmented sleep: you will fall asleep, but will wake up for 1-2 hours in the night only to fall back asleep. This lucky outcome will leave you pretty refreshed.
  • long night: on rare occasions, esp. if you are sleep deprived, early bedtime will result in a nice long refreshing night sleep.

Due to the precarious nature of Phase 15 sleep, it should rather be employed only in conditions of sleep deprivation, which provides good chances for a positive outcome. Otherwise, early bedtime may be unproductive at best, and bad for the quality of sleep, at worst.

Phase 18-24: Night sleep

If you try to nap in Phase 18-24, you are bound to trigger a normal healthy night sleep. This is ok as long as you do not get down to "napping" with the evil intent of stopping the process in 20-40 min. Here is where the pain of polyphasic sleeping becomes hardest to bear. As Dr Stampi noticed decades ago, it is not the problem with staying awake or with falling asleep that is most exasperating. The most painful part of a polyphasic life is when your brain wants to trigger the night sleep sequence and a polyphasic adept stubbornly disallows it (Stampi 1992[2])! This is as bad an interruption as any other abrupt stop to an all-or-nothing physiological process (urination, defecation, orgasm, swallowing, heartbeat, sneezing, coughing, childbirth, and the like). Many polyphasic bloggers note: "I noticed that when my naps get longer, I get groggy. So I try to keep them under 20 min". Duh! If you do not launch the night sleep sequence, you will not suffer the pain of interruption. Why nap in the first place then? It's easier to delay defecation than to stop it in the middle. The most unusual night-time nap control method I have encountered was... "I keep lots of junk in my bed. That keeps my naps short"!

Napping rules

Napping should occur at siesta time in Phase 7-8. Remember also that:

  • The napping phase is relative to the optimum natural waking time, not the actual waking time! For example, in a premature sleep, or in an interrupted sleep, an efficient nap can take place 10 hours from waking. If your sleep is not entirely natural, you need to take a correction for your circadian cycle that might be running in the background independently of your actual sleeping hours. Unfortunately, a vast majority of people with sleep phase problems have no idea how to efficiently measure and plot their natural cycle
  • The two alertness valleys are biologically dissimilar! Only the night-time valley can produce a typical long-drawn periodic NREM-REM interplay with a gradual increase in the proportion of REM. The subjective night period is marked by a characteristic increase in the release of melatonin. The length of siesta sleep, in biphasic sleep, is 4-20 times shorter than the natural night sleep. Phase response is elicited by stimuli that precede or follow the night sleep. However, the same stimuli may affect the timing of the siesta nap, which in turn may have an indirect impact on the sleep phase
  • Timing of naps will determine their structure. Some researchers believe that this can also affect the efficiency of naps for a particular type of mental task due to the fact that nap timing determines the proportion of NREM sleep to REM sleep. This might be true if you want to maximize value/time for specific tasks. However, with sleep optimizations, you should always go for the maximum total value. This is why the best nap is a nap well aligned with your midday circadian dip in alertness.

Good conditions for a nap are important. A nap in a semi-reclining position, or in a noisy room, or in bright lights, will also bring benefits to your alertness (on condition you actually manage to fall asleep, and perhaps pass Stage 1 NREM). However, a nap in a sleep-conducive environment will often last longer and be far more refreshing.

One nap per day is enough

Many people believe that every extra successful nap can be preciously helpful in restoring their mental energy. In a normal sleeper, who is not sleep deprived, an additional nap is indeed likely to bring increased alertness and improve mental performance. However, on a healthy schedule, all naps outside the siesta period should be very hard to accomplish. If the goal of sleep is defined as achieving maximum creative productivity, and if the night sleep can run its healthy course (i.e. there is no sleep deprivation), then any nap attempt at times other than the siesta time will be wasteful. This is because falling asleep should be difficult, and simply resting with the eyes closed does not yield a fraction of the neural benefit of an actual successful nap. Moreover, even if successful, an extra nap forced in in the morning is likely to interfere with the afternoon nap. Similarly, an evening nap may result in shortening of the night sleep. Those extra naps may bring incremental improvement in performance, but will reduce the overall efficiency of sleep and may cause ripples in the circadian system. Our biphasic nature makes it quite clear, we should strive at a single nap in the afternoon (in the 7th hour of waking). For some people, even this will be too much, and monophasic pattern is their optimum.

Many young creative individuals come up with their own designer sleep schedules. I often get mail with submissions of new sleep pattern propositions. For example, triphasic sleep: one main sleep episode of 6 hours (00:00-06:00) with two 30 minute naps after meals (12:00-12:30, and 18:00-18:30). Like most of artificial ways of making the sleep system work to design, this schedule is not likely to be efficient. Most people are strongly biphasic, and only biphasic or monophasic sleep works well for adults. However, if one throws away the second nap, the proposition will be pretty close to a natural biphasic rhythm: 0:00-6:00 and 13:00-14:00. Even though, designer schedules should always be avoided. The only exception is for designs that are an approximation of what SleepChart shows in free running sleep. As people differ in various parameters of their sleep control system, those who are very regular sleepers might indeed consider wiring a specific timing to their schedule as long as the timing is derived from their actual sleep pattern measurement. If sleep episodes in a designer schedule are not aligned with the circadian needs then they will often lead to a circadian chaos.

Conclusions: napping

If you want to sleep well and be productive, choose biphasic sleep, monophasic sleep, or free running sleep, whichever works best for you, and whichever you can afford.

Free running sleep synchronized with the daylight cycle is the healthiest form of sleep and will result in highest productivity

Once you run free, you will quickly determine if your prefer to sleep biphasically or monophasically. In creative lifestyle, biphasic sleep leads to higher productivity. See: Natural creativity cycle.

References

  1. Werth E., Dijk D.J., Achermann P., Borbély A.A., "Dynamics of the sleep EEG after an early evening nap: experimental data and simulations," American Journal of Physiology / Volume 271 / Issue 3 (September 1996): 501-510
  2. Stampi C. "Why We Nap: Evolution, Chronobiology and Functions of Polyphasic and Ultrashort Sleep." Boston: Birkhäuser, 1992.