So picking up where we left off, talking about the dangers and benefits of napping (or kipping, as they say in the commonwealths), a good question to ask first is, “how can you measure sleep?” Napping is a form of sleeping, but what type of sleep exactly?
Like we mentioned in the phases of sleep, some signatures of sleep are the brainwave activity as measured by electro-encephalogram (EEG). This requires contacts being placed on your head that can pick up the electrical current being produced by your neurons (as seen from the adorable baby below – he’s fine by the way, his parents enrolled him in Britain’s first ‘Baby-Lab’ to study infant brainwave patterns). This method will give a series of wave patterns to describe electrical activity across the brain.
It is literally the measurement of activity. In a set-up similar to the FitBit a transducer is attached to an individual’s wrist and their movement is recorded during sleep. This can be correlated with EEG or other measurements to give an idea of bodily function throughout the sleep cycle. It is also useful for assessing daytime sleep occurrences and diagnosing insomnia, circadian rhythm sleep disorders, and other conditions. Complex actigraphs can also record skin temperature, heartrate, ambient light, ambient sound, tremor, and galvanic skin response/resistance to paint a very detailed picture of what the user is experiencing, though most sleep actigraphs don’t employ all these inputs as they’re more useful for wakeful activity measurements. This approach is non-invasive and doesn’t restrain the user to a lab.
Next up there is polysomnography, which is by far the most comprehensive and inclusive set of readings about a sleeping individual. The result of the technique, a polysomnogram (PSG), is a medley of outputs including EEG, Eye-movement record, cardiac record (EKG), muscle record (EMG), and pulse oximetry and respiratory effort outputs for apnea diagnosis. It is appropriate for diagnosis of some but not all sleep disorders, particularly not for disorders leading to extreme fatigue. Preventative measures should be considered before some individuals undergo the test. This test requires many electrodes to be placed all over the body, and transducers to be placed over the oral cavities. A sleep technician must be present observing the measurements, and examining the patient. Scoring includes time to onset, efficiency, and time spend in each stage. The technician will also look for arousals, apneas, movements and restlessness. This approach provides a comprehensive temporally synced view at bodily activity in sleep.
Common Test Set-ups for Sleep
There are two final sleep disorder diagnostic tools I want to discuss briefly: Multiple Sleep Latency Tests (MSLTs) and Maintenance of Wakefulness Tests (MWTs). MSLT looks at the time it takes to fall asleep at different durations after a daytime nap – it distinguishes true sleepiness from disorders of excessive sleep. It utilizes the readings of a PSG to assess the state of wakefulness in patients. Meanwhile MWT measures a person’s ability to stay awake in a quiet, dark and nonstimulating room for a period of time. This test is usually given to a person receiving therapy for conditions causing daytime sleepiness which have been diagnosed. There are two periods of silent restful atmosphere in which a test subject must try to stay awake for 20-40 minutes. I would, without questions, always fail this test. Both of these tests are used to diagnose sleep disorders, but they help to paint the picture of what healthy sleep is and can help us frame whether or not napping contributes in a substantial way to positive outcomes.
NASA Says Naps Help
Now there are still other metrics of the effects of sleep and napping. For instance, there is a NASA study which looks at cognitive function on various sleep nap cycles and uses several tests to assess mental adroitness. “For vigilance and alertness, which involve the ability to maintain sustained attention and to notice important details, they found that the total amount of sleep during 24 hours remained the most important factor.”
Yet napping, especially in longer naps, was better at improving working memory; however the group mentions the phenomenon of “sleep inertia” or the persistence of grogginess after waking up particularly after naps, and offers little explanation. (Again, conventional wisdom asserts this is caused by REM cycle disruption in which an individual is woken at in inopportune time in their sleep cycle and whole stages are skipped).
Their final warning: napping to supplement short night-time sleep can enhance some performance metrics in the short term, but will eventually see diminished performance across the board without adequate sleep each day. Studies on this were discontinued however, I suppose since ‘Napping’ looked bad on their slashed budget reports.
To the Point
So it sounds like napping is good; and it can increase your working memory and supplement an insufficient amount of night-time sleep, but at some point you have to reset and get a full night’s sleep. However, for all those fancy sleep techniques and readouts I mentioned you don’t see a lot of work analyzing the quality of sleep you get during naps, and that would be very useful in helping to determine if they’re better than worse, and possibly how to nap more efficiently.
So what is the big deal again? Oh, right – the study of a British cohort that links napping with a higher hazard ratio of death. The survey study showed that individuals who napped less than an hour a day had an increased risk of dying (1.1x) independent of all other risk factors based on a 13 year follow-up study. And those who napped more than an hour each day? An even higher (1.2x) hazard ratio of death!
The primary cause was respiratory disease, but the paper makes no effort to causally link the two and so this mechanism of death-by-snoozing is completely unknown. Now while napping scientists everywhere search desperately for the missing control or the unintended bias of the results, the rest of us have to wonder how our naps might kill us (10-20% of the time).
So Are We Dead?
One issue in this paper did give me pause – they state pre-existing conditions did not bias the results, and they included sleep apnea as one such condition on a rather extensive list of covariates. However, apnea and other diseases (physical and psychological) could likely lead to difficulty in achieving a restful night’s sleep. This study could not assess vitals during sleep; neither could they track sleep latency, nor sleep efficiency (time to fall asleep, and time slept vs. time in bed, respectively) which are widely accepted indicators of sleep quality. And presumably, individuals with poor nighttime sleep quality from a preexisting condition would be more likely to need naps in the daytime.
Now, the paper did address this sort of biasing, but I would wonder if these heavy daytime nappers weren’t also the sort who suffered from some undiagnosed sleep-compromising condition already. If this were true, then the incidence of napping could be construed as a symptom and not a cause of the conditions leading to accelerated death (which might be no more comforting to us nap-takers out there!)
Finally the study has the weakness of all retrospective self-reported studies, and while the protocols were surely rigorous there is considerably variability in accuracy and standards of self-reporting.
So We’re Not Dead?
The correlation isn’t strong enough to make me worry, but beyond all that has anyone looked at how irregular sleep could even cause drastic physiological changes? In short not really. I’ll briefly describe two papers that tried to link sleep and disease by a chemical by-product.
One group looked at how disruption of circadian rhythms could potentially alter daily melatonin production, but they found that disruption depended strongly on an individual’s “chronotype” (morning vs. evening person). In this paper, male day and night shift workers were assessed for chronotype by a survey, and then had urine collected throughout shift and sleep. The samples were assayed for chemical changes in 6-sulfatoxymelatonin (6SM) which has been associated with longer sleep duration. At one point it was also believed to indicate lower breast-cancer risk.
This study showed ‘morning types’ on a night shift had more normal expression of 6SM than ‘evening types’ but that they could not draw links to cancer in their study. A group in China, however, was able to compare women diagnosed with breast cancer to controls and found that 6SM was in fact not even an indicator of breast cancer as previously believed, and sleep duration had no effect on outcome. So we still know nothing.
At some point in the future I’d love to circle back and tackle more about the history of circadian rhythm, entrainment, causes of sleepiness, instances of “microsleep” and important studies of developmental light avoidance in animals, but for now I think we can put the matter of napping to rest. As with wine, beer, running, fish oils, chocolate, sunlight and all the other wonderful things that are at once saving and killing us, I think we can cautiously go on napping in moderation and not risk imminent death (though we may endure temporary grogginess). The links are too tenuous now to draw any negative conclusions, and napping’s short term benefits are too real to deny, so I wouldn’t lose any sleep over it.