A short power nap (under 30 minutes, taken early in the afternoon) can temporarily improve alertness and cognitive performance without any demonstrable health risk. Longer or more frequent naps are associated with higher blood pressure, more cardiovascular disease and higher mortality, with genetic studies suggesting that this relationship is at least partly causal.
A short daytime nap, averaging around 55 minutes and ideally taken early in the afternoon (before 13:00), improves cognitive performance -- including alertness, memory and concentration -- for approximately two hours after waking. The effect is small to moderate (overall effect size 0.18, 0.29 for alertness) and holds regardless of age or sex. Note: virtually all research on this topic was conducted in a laboratory setting, not in a typical workplace. (PMID 34639511)
Immediately after waking, there may be a temporary period of sleep inertia: a groggy, foggy feeling that can actually impair performance briefly. Research findings on this are mixed, but the improved alertness only kicks in once this initial grogginess has passed. (PMID 34639511)
On the topic of health risks, a large meta-analysis of 44 cohort studies involving more than 1.8 million participants sends an important signal: naps lasting 30 minutes or longer are associated with higher rates of mortality, cardiovascular disease, and metabolic conditions such as diabetes. Naps shorter than 30 minutes, however, do not show this elevated risk. These are observational studies, so caution is warranted: people who nap for longer periods may already be in poorer health beforehand, meaning the nap itself may not be the cause. (PMID 39153335)
Two large genetic studies (together involving more than 800,000 participants from the UK Biobank) used Mendelian randomisation, a method better suited to establishing causality than conventional observational studies. Both studies found that a genetic predisposition to napping more frequently is associated with higher blood pressure and a larger waist circumference, even after adjusting for body weight. This strengthens the suspicion that a genuine causal relationship exists between regular napping and cardiovascular risk factors. (PMID 33568662, 37925459)
That same large meta-analysis also identified potentially beneficial effects: regular napping is associated with a lower risk of cognitive decline and muscle weakness (sarcopenia) in older age. However, the evidence for this is limited and, here too, cause and effect have not been established. (PMID 39153335)
Finally, avoiding daytime naps is often recommended as part of sleep hygiene, to improve night-time sleep. The direct evidence for this advice is, however, weaker than is commonly assumed; it has barely been tested outside clinical settings. (PMID 25454674)
Evidence for cognitive improvement comes from a meta-analysis of laboratory studies (PMID 34639511). Evidence for health risks comes from a large cohort-study meta-analysis (PMID 39153335) and two Mendelian randomisation studies in the UK Biobank (PMID 33568662, 37925459). Sleep hygiene advice is observational and limited (PMID 25454674).