Caught napping
We’ve All Heard The Gospel Of The Daytime Nap, With Benefits Ranging From Relaxation To Mood Improvements. But How Does It All Work? And Does It Work For All?
While there is still much that is unknown about sleep, its importance is well established. According to a summary of current research by the US’s National Institute of Neurological Disorders and Stroke (Ninds), “Everyone needs sleep, but its biological purpose remains a mystery. Sleep affects almost every type of tissue and system in the body – from the brain, heart and lungs to metabolism, immune function, mood and disease resistance. Research shows that a chronic lack of sleep, or getting poor-quality sleep, increases the risk of disorders including high blood pressure, cardiovascular disease, diabetes, depression and obesity.”
Naps, however, are an area of sleep that is even less understood. According to a 2017 study by researchers from the University of Massachusetts’ Neurosciences and Behaviour programme (NSB), titled Exploring the nap paradox: are midday sleep bouts a friend or foe?, “It is well established that in addition to reducing sleepiness, midday naps offer a variety of benefits: memory consolidation, preparation for subsequent learning, executive functioning enhancement and a boost in emotional stability.”
The paper says these benefits remain present even if a sufficient amount of sleep is obtained during the night before, and that daytime naps do not necessarily affect the quality of nocturnal sleep.
Sleep: The breakdown
To understand how naps benefit us and how long they should be, it is important to understand the two established primary stages of sleep, which we cycle through several times during a typical night. The first stage of sleep is non-REM (non-rapid eye movement) sleep, which is itself split into three stages. The first, which lasts a few minutes, is relatively light sleep, during which heartbeat, breathing and eye movements slow, and muscles relax, with occasional twitches. The brain waves also begin to slow from their daytime wakefulness patterns.
Heartbeat and breathing continue to slow in the second stage of non-REM sleep, and muscles relax even further. The body temperature drops; eye movements stop. You’ll spend more of your repeated sleep cycles at this stage than at other sleep stages through the night. The third stage of non-REM sleep is the deep-sleep stage that we all need, to feel refreshed. Heart rate and breathing slow to their lowest levels; muscles are at peak relaxation and brain waves are even slower. This stage is also known as slow-wave sleep (SWS).
The other primary sleep stage is known as REM (rapid-eye-movement) sleep. Here, “the eyes move rapidly from side to side behind closed eyelids. Mixed frequency brain-wave activity becomes closer to that seen in wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels. Most of your dreaming occurs during REM sleep. Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams,” says Ninds.
REM sleep typically comes at about 90 minutes after falling asleep, completing the initial sleep cycle.
Naps have a different physiology than a full night’s sleep. The NSB study found that naps in infants are indistinguishable from nocturnal sleep, as both are REM-rich.
Young adult naps, if of substantial length, will contain both non-REM and REM bouts, while naps of older adults are dominated by lighter non-REM stages, a short bout of SWS, and – less often – REM sleep.
How often should I nap?
The length and frequency of a nap are not exactly standard, and neither are the lengths of naps that have been researched. A study by the University of California published in 2012, titled The effect of nap frequency on daytime sleep architecture, takes a closer look at how the frequency of naps affects their quality.
The researchers studied 27 healthy non-smoking students between the ages of 18 and 35, who all took a series of 75-minute naps. They found that “The more individuals nap during a week, the greater the amount of Stage 1 and Stage 2 [non-REM] sleep and the less SWS [deep sleep] in their naps… Subjects in the one to two nap [per week] frequency bin had significantly more Stage 2 sleep than subjects who took three to four naps [a week] or subjects who took zero naps.”
Within non-REM sleep, data showed a progression from deep sleep to lighter and lighter stages of sleep based on nap frequency – people who did not nap during the prior week had the highest amount of deep sleep, people who took one to two naps had the most Stage 2, and people who napped the most (three to four naps) had the most Stage 1 sleep.
Keep in mind that SWS is the deep sleep stage where the muscles are fully relaxed, and where it might be more difficult to wake someone up. Waking up at this phase is also more likely to result in waking up feeling groggy and tired, negating some of the benefits of a nap. The study found that non-nappers, or non-frequent nappers, “may be more inclined to avoid napping due to high levels of SWS that produce unpleasant after-effects”. However, according to the research, this is something that may improve, the more frequently one naps.
Great! So how long should I nap? In 2006, Australia’s Flinders University conducted research to measure the efficacy of different nap durations on subjects who had slept up to only five hours the previous night. The subjects were 24 “healthy, young adults who were good sleepers and not regular nappers”.
The researchers found that the 10-minute nap produced immediate improvements in all outcome measures, including sleep latency, subjective sleepiness, fatigue, vigour, and cognitive performance, with some of these benefits maintained for as long as 155 minutes. This makes the 10-minute nap overall “the most effective afternoon nap duration of the nap lengths examined in this study”.
Research from subsequent studies, published by the American Sleep Association, found that for most people a good nap is between 15 and 30 minutes, with some doing better at 10 minutes, and others at 40 minutes. This way, nappers can avoid crossing over into deep sleep, and therefore not feel groggy when they wake.
Is that all? Can I go nap now?
While research suggests that napping is great for children, and recommended for young as well as middle-aged adults, the current literature on its effects on older populations is inconsistent.
The 2017 NSB study, for example, was motivated in particular by other studies that associated napping “with a number of subsequent negative outcomes”, including increased risk for hypertension, depression, diabetes, increased mortality and cognitive decline. These have predominantly been identified in older adults but also exist in middle-aged and young adults.
After aggregating findings from previous research, the authors of the study conclude that when it comes to older populations, “excessive napping has been linked with negative outcomes. Yet there is no direct evidence suggesting that midday napping is detrimental. Although it is unlikely that essential naps are causal in inducing comorbidities, it is perhaps premature to make this statement definitively. Therefore, it is also premature to prescribe napping in this population.”
They are not the only ones to highlight the inconsistencies. A 2016 study in the Journal of the American Geriatrics Society titled Afternoon Napping and Cognition in Chinese Older Adults notes that “a few studies have reported that longer napping duration is associated with cognitive impairment, but other studies have found beneficial effects of napping on cognitive function”. Looking at subjects 65 years and older, the study concluded that “moderate nappers had better overall cognition than non-nappers or extended nappers. Non-nappers also had significantly poorer cognition than short nappers.”