Research has turned up all sorts of insights into how sleep affects our bodies, and now a new study has a little more to add: an afternoon nap might boost mood, memory, and other cognitive functions, but is also associated with higher glucose levels.
The new research looked specifically at adolescent students – a group that's known for its erratic sleep patterns. A total of 59 students aged 15-19 were restricted to 6.5 hours sleep every 24 hours, with half banking that in one continuous go at night, and half introducing a 90 minute nap on top of 5 hours of overnight sleep.
Based on tests carried out during the day, the afternoon nappers scored higher in several areas you would associate with better health: feeling more positive, feeling less sleepy, and doing better at various memory and cognitive tasks.
"Interestingly, under conditions of sleep restriction, students in the split sleep group exhibited better alertness, vigilance, working memory and mood than their counterparts who slept 6.5 hours continuously," says one of the researchers, neuroscientist Michael Chee from the Duke-NUS Medical School in Singapore.
"This finding is remarkable as the measured total sleep duration over 24 hours was actually less in the former group."
Before you start planning a similar 6.5-hour sleep schedule for yourself, split into two shifts, note this – based on an earlier 2016 study, the researchers found performance and mood was worse overall compared with people banking 9 hours of shut-eye each night.
And as we mentioned earlier, blood glucose levels were higher in the group who slept in shifts – that's one risk factor for type 2 diabetes. There was no major difference in blood glucose levels between the group that slept continuously for 6.5 hours and the group from the 2016 study that banked 9 hours of sleep a night.
That's an important distinction, because while some previous research has suggested a regular nap time might be beneficial on top of overnight sleep, few studies have looked at the metabolic as well as the cognitive impact.
This metabolic response was a particular focus for the researchers behind the new study, given that we already know there's likely to be an association between a lack of sleep and the risk of developing diabetes.
While we're talking about a small and limited sample here, it's possible that napping drives glucose levels higher than they would be if all our sleep was banked at night, even if mood and cognitive performance gets a boost.
Ultimately though, the study concludes that getting an inadequate amount of sleep every 24 hours isn't good for you, however you split it – especially for adolescent brains that are still developing. For this age group at least, an average of 9 hours of slumber a night is best.
"They are best advised to obtain the recommended amount of nocturnal sleep," conclude the researchers.
The research has been published in Sleep.