Napping

What it is

A nap is a short sleep taken during the day, outside the main night's sleep. Trials tested naps in the afternoon and during night shifts, and the average nap in one meta-analysis lasted 55 minutes [1][2]. Naps of 10 to 90 minutes were studied, and a 2-hour nap opportunity was tested too [3][4][5].

How it might work

Proposed, not established: a nap may ease sleep pressure, which could explain the quick lift in alertness after a short sleep [6]. Daytime naps mostly contain non-REM sleep with slow waves, and slow waves and sleep spindles are thought to help the brain store new memories [7].

What the research says

Alertness and thinking. A meta-analysis of 11 studies with 381 people, mostly run in labs, found that people who napped improved on cognitive tests after the nap, with the largest gain in alertness [1]. In 105 interns after night shifts, 15-minute and 45-minute naps improved cognitive tests in the mid-afternoon compared with no nap [2]. In 28 people on a simulated night shift, a 2-hour nap was followed by better alertness and mood than a 30-minute or no nap [5].

Memory. In 50 young adults, a 90-minute nap helped them recall more words than quiet rest, but in 45 older adults it did not [8]. In 32 young adults, only a 30-minute nap improved memory encoding; 10- and 60-minute naps did not [3].

Mood and sleepiness. In 32 young adults, naps of 10 to 60 minutes raised positive mood and reduced self-reported sleepiness for up to four hours [3]. In 111 adolescents with short sleep, a 30-minute lunchtime nap lowered sleepiness and kept positive mood up compared with quiet rest [9].

Sleep loss. The results are mixed. In 280 people kept awake overnight, a 30- or 60-minute nap did not reduce the drop in performance [6]. In 9 judo athletes after partial sleep deprivation, a 20-minute nap increased peak sprint power compared with placebo [10].

Heart disease. Results are mixed. Long naps of an hour or more were linked to higher cardiovascular disease risk in one meta-analysis (RR 1.82), while a later pooled analysis of cohorts found no overall link [11][12].

Death, diabetes and cancer. A meta-analysis of 44 cohort studies with 1,864,274 people found habitual napping linked to higher all-cause mortality, with naps of 30 minutes or longer linked to higher risk and shorter naps not [13]. Another meta-analysis found a higher death risk for naps of 1 hour or more (HR 1.22) but not for naps under an hour (HR 1.00) [14]. A meta-analysis of 7 cohorts with 249,077 people found habitual nappers had 17% higher type 2 diabetes risk [15]. In one UK Biobank cohort of 460,923 people, frequent napping was linked to slightly higher overall cancer risk (HR 1.03 to 1.04) [16].

Brain health with age. The cohort meta-analysis linked habitual napping to lower risk of cognitive impairment [13]. An observational review of 35 studies found that in older adults, short and moderate naps were linked to better cognition than no nap or long naps, while intervention results were inconsistent [17].

Doses used in studies

Trials tested naps of 10, 15, 20, 25, 30, 45, 60 and 90 minutes, plus a 2-hour nap opportunity [3][2][10][18][19][20][5]. These are study conditions, not recommendations.

Safety and interactions

Common side effects. The main downside reported is sleep inertia, a groggy feeling right after waking. In 31 adults, a 30-minute nap but not a 10-minute nap was associated with it [21]. In 28 emergency medical workers, the drop in performance after 30-minute and 2-hour naps faded within 10 to 30 minutes [22]. Other side effects were not reported in the trials read [21].

Upper limits. No upper limit for nap length was identified in the studies read. Long naps are linked to higher risk of death in observational studies [14].

Medicines. No data on interactions with medicines were found in the studies read.

Pregnancy and breastfeeding. One systematic review says napping may be a risk factor in pregnant women [23]. No data on breastfeeding were found.

Health conditions. Daytime napping is common in depression and bipolar disorder, and may be a risk or prognostic factor there, so it deserves clinical assessment [23]. No data were found for people with kidney, liver or bleeding disorders.

Daytime sleepiness. Frequent daytime sleepiness and napping can signal a sleep disorder or another underlying problem, so people who nap often may want to discuss it with a doctor [23].

Caveats and open questions

Most trials are small, with 9 to 111 people each, and many were run in labs or with athletes and students. Results may not carry over to offices or home life [1][18]. The benefit for memory was seen in young people, and the older adults in one trial showed no benefit [8].

Cohort studies cannot show that naps cause disease. The authors of one cohort meta-analysis note residual confounding and reverse causality, meaning illness may lead to napping rather than napping causing it [15]. Whether the death risk comes from napping itself or from long sleep overall remains debated [24].