What does a daytime nap do to your health?

A short nap of less than 30 minutes has a measurable positive effect on alertness and cognitive performance. The effect lasts up to two hours after the nap, and napping early in the afternoon (before 13:00) works best. Immediately after waking you may feel briefly groggy, but that grogginess passes quickly. That said, most studies on this topic were conducted under laboratory conditions, not in ordinary work settings.

People who regularly nap for longer than 30 minutes show a different picture. Large observational studies point to a higher risk of mortality, cardiovascular disease, metabolic disease and cancer. A meta-analysis of more than 1.8 million people found this pattern exclusively with naps of 30 minutes or longer, not with shorter naps. Naps longer than 30 minutes are also associated, in separate research, with a 7 to 20% higher risk of type 2 diabetes.

The question is whether napping for long periods is itself unhealthy, or whether people who are already ill simply tend to nap more often and for longer. Two large genetic studies (together covering more than 800,000 British participants) suggest that napping regularly does indeed contribute to higher blood pressure and a larger waist circumference, independent of other factors. That points more towards a causal relationship than coincidence, though caution is still warranted here too.

The same large meta-analysis also found, strikingly, some positive signals: regular nappers had a lower risk of cognitive decline and muscle loss. That finding is observational and warrants further research.

On the sleep-hygiene recommendation to avoid naps in order to improve your night-time sleep: this is less well supported than you might expect. An association has been found between daytime napping and poorer night-time sleep, but experimental evidence that avoiding naps actually improves your sleep quality is largely absent.