Napping and Human Sleep

By Victor

“Blue Zones”

This is the name given by epidemiologists to places where people tend to have extraordinary lifespans.  Sardinia (an Italian island), Ikaria (a Greek island), Okinawa, the Nicoya peninsula of Costa Rica, and Loma Linda California are such Blue Zones.

The Ikaria Study looked at nearly two hundred subjects, men and women, over the age of eighty in order to identify modifiable risk factors associated with longevity.  Lifestyle questionnaires revealed that the majority remained physically active.  They also consumed large amounts of olive oil, fruits, vegetables and fish; they ate little red meat and very few sweets.  None of this is very surprising.  However, one discovery was unexpected.  Eighty four percent of the men and sixty-seven percent of the women over age eighty took daily naps.  This result is consistent with two earlier Greek studies.  The MEDIS study reported that of nearly 12,000 subjects, virtually all over the age of ninety took daily naps.  The EPIC cohort study of nearly 24,000 subjects over a six year period found that “those occasionallynapping had a 12% lower coronary mortality, whereas those systematically napping had a 37% lowercoronary mortality”. 

These results are surprising because they contradict other research showing increased mortality associated with napping.  See:  Don’t nap!, Daytime napping and mortality, Napping, Nighttime Sleep, and Cardiovascular Risk and Napping Is Associated with Increased Risk of Type 2 Diabetes.

Why would napping have such adverse metabolic and cardiovascular effects, increasing the risk of mortality?  How can we reconcile such striking contradictions in the epidemiological data?  Is napping beneficial or detrimental to health and longevity?

Sleep is a physiological process observed in many diverse species including invertebrates.  While all mammals sleep, only primates exhibit consolidated (monophasic) sleep.  All other mammals display fragmented sleep, a series of brief, episodic sleep periods.  Interestingly, marine mammals, such as dolphins, sleep with one hemisphere of their brain at a time, always keeping the alternate hemisphere awake, presumably to maintain respiratory and sensory functions while swimming.(ref)  The fact that cetaceans have developed such an unusual form of sleep emphasizes the indispensability of this universal phenomenon.  Sleep is known to serve many important purposes, including memory consolidation, improved cognitive function, motor skills, etc.  Given the many beneficial effects of sleep, and considering that periodic sleep is the norm in other advanced mammals, it is quite surprising that the additional sleep associated with a daily nap would have harmful consequences. 

Reduced sleep has adverse consequences.

There is no controversy about the adverse health consequences of reduced sleep.  Both population and clinical studies have consistently reported increased mortality associated with reduced sleep. 

See, for example the November 2010 blog entry Sleep and longevity, Sleep Duration and All-Cause Mortality and Habitual Sleep Duration and Insomnia and the Risk of Cardiovascular Events and All-cause Death

Reduced sleep increases mortality through multiple pathways.  It impairs immune function making us more susceptible to diseases.  It increases systemic inflammation provoking hypersecretion of proinflammatory cytokines, such as NFK and IL6.  Reduced sleep disrupts normal circadian function which is involved in the regulation of virtually all physiological processes in the human body.  Reduced sleep disrupts normal secretion patterns of growth hormone, cortisol and sex hormones, as well as metabolic adipokines, such as leptin, ghrelin, etc.  In addition to neurological degeneration and cardiovascular diseases, reduced sleep is associated with diabetes, obesity, and even increased risk of cancer.   See Metabolic, Endocrine, and Immune Consequences of Sleep Deprivation, How (and why) the immune system makes us sleep, Sleep and Metabolism: An Overview, Role of Sleep and Sleep Loss in Hormonal Release and Metabolism, and Short duration of sleep increases risk of colorectal adenoma.

Sleep Fragmentation

Sleep continuity appears to be just as important as total duration.  Human sleep follows a complex pattern of stages which results from the dynamic interaction of multiple thalmic and cortical oscillators.(ref, refAwakenings interrupt this normal progression of stages.  Brain imaging has shown that patients who suffer from sleep fragmentation due to obstructive sleep apnea (OSA) actually have “a reduction in grey matter volume in the right middle temporal gyrus compared with non-apnoeic controls. A reduction in grey matter was also seen within the cerebellum, maximal in the left lobe VIIIb close to XI, extending across the midline into the right lobe. These data show that OSA is associated with focal loss of grey matter that could contribute to cognitive decline. Specifically, lesions in the cerebellum may result in both motor dysfunction and working memory deficits, with downstream negative consequences on tasks such as driving.”(ref)  In healthy subjects, sleep fragmentation also disrupts normal neuroendocrinal secretory patterns causing adverse metabolic effects(ref), and impaired cognitive function.(ref)

Does napping reduce nighttime sleep?

In humans, reduced sleep duration (or continuity) is associated with the same adverse consequences reported in some epidemiological studies of napping, namely obesity, metabolic syndrome, cardiovascular disease, etc.  This suggests one way napping could potentially have adverse effects — by reducing the duration or quality of nighttime sleep.  Indeed, there is evidence to suggest that this is exactly what happens.

Napping, Nighttime Sleep, and Cardiovascular Risk Factors: “More frequent napping was associated with shorter nighttime sleep duration averaged across the nine nights of actigraphy (especially among African Americans), more daytime sleepiness, more pain and fatigue by diary, and increased body mass index and waist circumference. Shorter nighttime sleep duration was associated with taking a nap during the next day and taking a nap was associated with less efficient sleep the next night.”

Does napping have any benefits?

A countermeasure for sleep loss.  Just one night of reduced or fragmented sleep can impair cognitive function.  A nap the following day can reverse these effects.

Daytime napping after a night of sleep loss decreases sleepiness, improves performance, and causes beneficial changes in cortisol and interleukin-6 secretion: “Sleep loss has been associated with increased sleepiness, decreased performance, elevations in inflammatory cytokines, and insulin resistance. Daytime napping has been promoted as a countermeasure to sleep loss. . . In summary, a 2-h mid-afternoon nap improves alertness and, to a lesser degree, performance and reverses the effects of one night of sleep loss on cortisol and IL-6 secretion. The redistribution of cortisol secretion and the prolonged suppression of IL-6 secretion are beneficial, as they improve alertness and performance. Improved alertness and performance were associated with increased cortisol and decreased IL-6 levels, suggesting their roles as arousal and sleepiness factors, respectively, in humans.”  (Interleukin-6 is a pro inflammatory cytokine.)

Daytime napping: Effects on human direct associative and relational memory: “Participants who napped demonstrated significantly better retention of direct associative memory, as well as better performance on a surprise task assessing their relational memory, in which participants had to associate the two faces previously paired with the same object during learning. Particularly noteworthy, relational memory performance was correlated with the amount of NREM sleep during the nap, with only slow-wave sleep predicting relational memory performance. Sleep stage data did not correlate with direct associative memory retention. These results suggest an active role for sleep in facilitating multiple processes that are not limited to the mere strengthening of rote memories, but also the binding of items that were not directly learned together, reorganizing them for flexible use at a later time.”

The effects of sleep on memory and learning are well-established.  However, it appears that there is a window of time, in which sleep results in optimal recall and learning.  For a more detailed discussion, see Delayed Onset of a Daytime Nap Facilitates Retention of Declarative Memory.

The effects of napping on cognitive functioning: “Naps can reduce sleepiness and improve cognitive performance. The benefits of brief (5-15 min) naps are almost immediate after the nap and last a limited period (1-3h). Longer naps (> 30 min) can produce impairment from sleep inertia for a short period after waking but then produce improved cognitive performance for a longer period (up to many hours). Other factors that affect the benefits from the nap are the circadian timing of the nap with early afternoon being the most favourable time. Longer periods of prior wakefulness favour longer naps over brief naps. Those who regularly nap seem to show greater benefits than those who rarely nap. The significant benefits of a brief nap, containing virtually no slow wave EEG activity, are not predicted by the present theory of homeostatic sleep drive. A new biological process suggests that sleep onset followed by only 7-10 min of sleep can result in a substantial increase of alertness because it allows the rapid dissipation of inhibition in the ‘wake-active’ cells associated with the ‘sleep-switch’ mechanism.”

Reconciling the contradictory data.

Napping is associated with many benefits, including improved alertness, mental function, and is associated with improved health and increased longevity in some epidemiological studies.  Napping can also interfere with healthful nighttime sleeping patterns.  This is likely the result of disruption of normal circadian periodicity.  Reduced duration or quality of nighttime sleep likely accounts for the contradictory epidemiological data.  Although most questionnaires do not contain precise data on nap duration, nap length appears to be critical in determining the effects on nighttime sleep.  Short naps can provide the benefits without disrupting normal, circadian, nighttime sleep.

Good sleep, bad sleep! The role of daytime naps in healthy adults: “A nap during the afternoon restores wakefulness and promotes performance and learning. Several investigators have shown that napping for as short as 10 min improves performance. Naps of less than 30 min duration confer several benefits, whereas longer naps are associated with a loss of productivity and sleep inertia. Recent epidemiological studies indicate that frequent and longer naps may lead to adverse long-term health effects. A nap of less than 30 min duration during the day promotes wakefulness and enhances performance and learning ability. In contrast, the habit of taking frequent and long naps may be associated with higher morbidity and mortality, especially among the elderly. The benefits of napping could be best obtained by training the body and mind to awaken after a short nap.”

Older individuals typically have a reduced nighttime sleep phase, likely due to age-related deterioration in the dopaminergic system and circadian rhythmicity.  Given the critical importance of sleep for neurological function, this reduced sleep may result in a self-reinforcing cycle of neurdegeneration.  See:  Perspective on Sleep and Aging  In this regard, it is interesting to note that in the Greek studies, the benefits of napping seem to increase with age.  In the population studies showing negative effects of napping, the minimum age of participants was lower by several decades (40-50 y).  In one recent study on older subjects (m = 70y), even naps of 2hrs had no detrimental effect on nighttime sleep length or quality.  It is also noteworthy that the 2hr naps had no increased benefit over 45min naps.  Subjects were also better able to comply with the shorter nap duration.

Effects of a Month-Long Napping Regimen in Older Individuals: “These results support and extend our previous findings (10) that a daytime nap may improve neurobehavioral functioning in healthy older adults, without negatively affecting subsequent nighttime sleep. Whereas, our previous study examined the effects of a single, 2-hour nap opportunity on performance and nighttime sleep quality, the current study focused on the impact of a longer-term napping regimen, and on the relative effectiveness of a short (45 minute) versus a longer (2 hour) nap opportunity. Because the study was conducted to examine the effectiveness of napping as a possible countermeasure against some of the negative consequences of age-related changes in sleep, we were also interested in the degree to which subjects could adhere to the napping regimen.

There were few differences between the Short and Long nap groups in terms of their impact on nighttime sleep and daytime sleepiness. Both Short and Long nappers showed significant increases in 24-hour sleep amounts when compared to their baseline sleep times, and neither the short nor long nap had a negative impact on nighttime sleep: sleep onset latency, sleep efficiency, and sleep architecture remained essentially unchanged across the study. This finding is in agreement with a majority of studies that have examined the relationship between napping and subsequent nighttime sleep quality in older individuals (611; 2125) and argues against the frequently-expressed notion that naps should be avoided. The increase in total sleep time per 24-hours resulted in decreased daytime sleepiness . . . Our findings may indicate that the beneficial effects of napping on neurobehavioral function may not be maximized even after a month of scheduled naps. It is conceivable that the negative effects of years of chronic sleep restriction associated with age-related truncation of nighttime sleep may require a longer time to be fully mitigated. . .From the current data, it is of course not possible to evaluate whether naps shorter than 45 minutes would also be associated with enhanced waking function, though at least one study has reported beneficial effects, in a small group of habitual nappers, of a 30-minute nap (7).”


Evidence indicates that we may live longer, and enjoy the benefits of napping, including enhanced learning, and improved cognitive function without interfering with nightime sleep, provided we limit the length of our siestas.  I recommend napping no more than thirty minutes for young people, and no more than one hour for the elderly.  Early afternoon timing provides optimal benefits, while minimizing circadian disruption.  If you have trouble falling asleep in the afternoon, if feasible, you may want to try a hammock.  Research shows that rocking motions not only hasten sleep onset, but improve sleep quality, inducing more rapid deep sleep.(ref)  Finally, avoid sleeping too long!  Although we should all try to get 7-8h of uninterrupted, nighttime sleep, studies have shown that sleeping more than 8h per night reduces cognitive performance.  One study, of more than 5,000 subjects, showed that sleeping more than 8h or less than 7h over a two year period, both resulted in a cognitive impairment equivalent to a 4-7y increase in age.(ref)

See, also: Benefits of napping in healthy adults: impact of nap length, time of day, age, and experience with napping, and Naps, cognition and performance.


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