It’s one of those frustrating situations where the truth is out there but the evidence is unclear: napping can be restorative and energizing, but it might lead to your untimely death. Recently a collection of papers have come out which argue that certain types of daytime sleep are beneficial, while another paper found an incidence of earlier death with those who napped (these results were sensationalized in a nice piece on Gothamist with the headline “Soul-Crushing Study Claims People Who Nap Risk Early Death“). This article summarizes a paper in a scientific journal – we’ll get into the specifics later, but the spoiler is that there are many reasons not to be too paranoid about nap-induced fatality.
In a classic science conundrum we must decide whether napping gives us new life or rushes us into the grave. Luckily, there is a wide field of sleep science and studies of the physiology of napping and catching up with more REM sleep; the concept of how to sleep has been a philosophical focus for thousands of years, and our empirical understanding of sleep, circadian rhythms, and bodily recovery have been growing since the 1930s; so then we, as a pretty darn clever group called humanity, must have a good idea of why we sleep and how to do it better, right?
Before I answer that question I’ll disclose that this author is a chronic, faithful, and usually accidental napper. My personal experience is that waking before sunrise and staying up late into the night is the best way to get the most out of life, but the cost is not sleeping enough at night. And while most aspects of graduate school life keeps you on your toes, occasionally you’ll be in the middle of a PCR (an experiment designed by brilliant biologists to drain the life out of younger biologists) and browsing a reference paper when your eyelids close; and when you wake up your PCR is 20 minutes closer to analysis, your face has indelible and inscrutable lines forced into it, and your reference paper is covered in saliva – and typically you will feel good about the whole affair.
Now, to the science – despite millennia of observing sleep, what we know for sure can be broken into a few points courtesy of the Harvard Sleep Lab :
- Sleep is a period of reduced physical activity
- There is a typical posture of sleep (eyes closed, horizontal, etc)
- We have reduced responsiveness to external stimuli during sleep
- Sleep is transient (compared to hibernation, comas, and other lapses in consciousness)
- Sleep is accompanied by great variation in physiological outputs (blood glucose, brainwave activity, core temp) especially during REM (rapid eye movement sleep).
Further changes occur as well – most are associated with organs reducing their activity and using less energy. However, there is a noted increase in the release of Growth Hormone which indicates sleep is essential for growth, repair, digestion and other such processes. In general sleep is believed to allow for muscle repair and generation, energy conservation, restoration of many organ systems including our immune system, and to results show sleep plays some role in brain plasticity and memory consolidation.
So sleep is essential. But that doesn’t get at the ‘kind’ of sleep that is essential or the other kind that might have you one pajama footie in the grave. The conventional wisdom, the internet and even peer reviewed literature warns of oversleeping, undersleeping, sleeping too lightly, sleeping in phases, and how sleep lost cannot be easily regained but rather that the damage is somehow permanent. Experts will say quality, quantity and consistency of sleep are the most important factors, but beyond that advice it becomes difficult to quantify.
Quickly, what are the stages of sleep? There are four stages leading to REM, the hallmark of deep sleep. Stage 1 is light sleep; stage 2 is slow brainwave sleep; stage 3 is extremely slow delta wave sleep with intermittent bursts of brain activity; stage 4 is uninterrupted deep delta wave sleep. Stages 3 and 4 in some literature has been combined, and it is where sleepwalking, bed-wetting, and night-terrors may occur. REM sleep is punctuated by eye motion, brainwave activity, and dreams. Young children spend half their sleep in REM, and the older we age the less time we spend in REM. Could there be a correlation between growing smarter, staying young, and consolidating memory and REM sleep?
One more term to define is a circadian rhythm, from the latin circa (around) and dia (day), and not, as my mother calls it, a cicadian rhythm which is presumably the beat to which insects chirp in the summer. To qualify as one such rhythm the cycle must be a naturally occurring or endogenous reaction that has a period of about 24 hours and can be entrained by alteration of stimuli, namely light and temperature. It is this set of impulses that controls hormones, metabolism, and of course sleep.
So one question scientists ask is, “what do our bodies want?” This is a difficult question, because it is nearly impossible to uncouple our bodies from our conscious minds and our obligations and stimuli. It is difficult to empirically reason away from a 24-hour cycle since humans evolved on earth and the planetary day has not changed considerably to our knowledge in that time span. However, free-running sleep experiments can indicate what the human circadian rhythm prefers. Further, some groups set to ‘disentrain’ subjects from a preferred sleep cycle by removing external cues altogether. So, upon doing such an experiment, what would we expect? A 2007 paper in the Journal of Sleep Research notes, “polyphasically-placed sleep is the rule, rather than the exception, across the entire animal kingdom.”
This is an observation of the broader animal kingdom, and organisms’ tendency to sleep in spurts as needed (polyphasic meaning in ‘many forms’), but also specifically with longer “anchor sleep” in their preferred resting time (night for some, daytime for others). Mid afternoon napping is the most common example of human bi-phasic sleep, with an anchor sleep at night, and either a short power nap, or a longer 90-minute full-cycle nap during the day. Any other length of nap is believed to include a disruptive REM cycle fraction. The tendency to feel sleepy in the mid afternoon is termed a ‘circadian trough’, and it widely occurs for humans around 2pm.
The 2007 study found subjects slept less the older they got, but after 6 hours of wakefulness subjects would drift to a sleep episode regardless of age. This makes an interesting parallel with other studies showing the polyphasic sleep patterns of infants, and sheds light upon the developmental factors that change those patterns in human adolescence and adulthood. Further, the study posited that sleep occurred in either Major Sleep Episodes (MSE, 4+ hours) or Naps, and while most sleep was in a nighttime initiated (8pm-8am) MSE, or a daytime initiated (8am-8pm) nap, elderly subjects experienced more nighttime naps, and exclusively younger people fell into daytime MSE sleep. But the real take-away for us is that people will, in the absence of stimuli, undergo polyphasic sleep. Also this study was recorded by laboratory technicians and was not self reported, making it a far more reliable result than, say, that British nap study.
In my next entry we’ll look into some measurements of sleep science (Multiple Sleep Latency Tests, Maintenance of Wakefulness tests, actimetry, polysomnography, and some brain scans) and a few more studies conducted in the last 100 years. Then, when we know so much about sleep it’ll haunt our dreams, we’ll try to poke some holes in the papers that hate napping so bad.
And a parting note for this week: the word “nap” itself was probably coined around the 12th century, and is a derivative of the high German hnaffezen, meaning to doze. Its occurrence in literature and publication has spiked since the ‘60s and its use is at an all time high today (in the period since 2010), whereas words like ‘doze’ and ‘siesta’ are both down, according to Google. ‘Sleep’, is also up, but modestly compared to ‘nap’.