A short nap may be healthy, but sleeping longer than 30 minutes may mean problems are on the way.
These are the findings of research being presented at the American College of Cardiology's (ACC's) 65th Annual Scientific Session in Chicago, IL.
Sleep is important for health, and statistics suggest that many people do not get enough sleep. The Centers for Disease Control and Prevention (CDC) estimate that over 25% of people in the US lack sleep, and that 10% experience chronic insomnia.
In 2015, Dr. Tomohide Yamada, PhD, a diabetologist at the University of Tokyo in Japan, and colleagues produced evidence linking naps of 60 minutes or more to an 82% increase in cardiovascular disease and a 27% greater risk of all-cause mortality.
They also demonstrated that the risk of diabetes was 46% higher among those who napped for longer than 1 hour - and 56% higher among those who felt excessively tired.
However, the likelihood of each condition dipped when people napped for less than 30 minutes.
Steep rise in metabolic syndrome in those who nap for over 40 minutes
In the current study, Dr. Yamada led a meta-analysis of 21 studies involving 307,237 participants from Asia and the West.
They collected data about daytime sleepiness and napping through questions such as, "Do you have a problem with sleepiness during the day?" or "Do you take a daytime nap?" or "Do you sleep during the day?"
A J-shaped relationship emerged between napping and metabolic syndrome risk. People who spent less than 40 minutes napping had no increased risk for metabolic syndrome, but beyond 40 minutes, the odds increased sharply.
Napping for 90 minutes correlated with a 50% higher chance of metabolic syndrome, as did excessive daytime tiredness. Napping for less than 30 minutes appeared to decrease the risk.
No relationship has emerged between time spent napping and obesity, although obesity is linked to diabetes and metabolic syndrome.
Dr. Yamada says:
"Sleep is an important component of our healthy lifestyle, as well as diet and exercise. Short naps might have a beneficial effect on our health, but we don't yet know the strength of that effect or the mechanism by which it works."
Further study needed
The researchers call for further studies to confirm the findings and to gain a better understanding of how sleep habits influence metabolic syndrome and cardiovascular disease.
They suggest focusing on how short naps could improve cardiovascular health and on the interaction between long naps, daytime sleepiness and metabolic syndrome.
They hope that in future, napping habits could help predict other health problems.
Limitations include the fact that the subjects were not representative of the global population and that napping times were self-reported. The authors propose measuring sleep time in a lab or with a sleep tracker.
The National Sleep Foundation suggest that naps of 20-30 minutes can improve alertness without leading to further tiredness.
Medical News Today has previously reported that waking early on working days may increase the risk of metabolic disease.