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Suspend your skepticism for a moment, and imagine a powerful new performance aid that boosts your physical power, improves your reaction time and decision-making, and protects you against oxidative stress and muscle damage.

In exchange for those benefits, would you be willing to tolerate a few minor side effects – such as, say, bed head? That’s the bargain offered in a new study of midafternoon naps, published in the International Journal of Sports Physiology and Performance by a research team at universities in Tunisia and France. The findings add fuel to a continuing debate about the performance benefits of napping, but they also offer a reminder that getting it right isn’t as simple as it seems.

The study involved nine judo athletes, who completed a series of tests under four different conditions: after a night of regular sleep; after a night where sleep was restricted to 4.5 hours; with sleep restriction, plus a short 20-minute nap; and with sleep restriction, plus a longer 90-minute nap. The outcome measures involved a series of short sprints, computer-based reaction and decision tests, and blood sampling to measure muscle damage and oxidative stress.

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On the surface, the results were fairly straightforward: A night of short sleep hurt all the performance and health measurements, and an afternoon nap partly or fully restored them.

That’s what you’d expect based on previous research – but there are some interesting new wrinkles, according to Anthony Blanchfield, a psychophysiologist at Bangor University in Britain, who has studied napping in athletes and was not involved in the Tunisian study. “The finding that napping in athletes can specifically attenuate some of the markers that are linked to oxidative damage and muscle damage is novel,” he wrote in an e-mail.

Over the past few decades, sleep researchers have accumulated evidence that getting less sleep than you need has effects throughout the body, pumping up stress hormones, suppressing immune function, and promoting systemic inflammation that raises the risk of cardiovascular disease and diabetes. But whether a brief afternoon nap is enough to reverse these negative effects remains unclear.

One key aspect of the Tunisian study is that the naps came after a night of partial sleep deprivation. That might be applicable if, for example, you’ve had a restless night before a big competition, but it’s less clear that the nap would have similar benefits after a normal night of sleep.

In a 2018 study led by Blanchfield and his colleague Sam Oliver, runners completed a pair of time-to-exhaustion tests on a treadmill, once with a 40-minute nap opportunity finishing 90 minutes beforehand, and once without. The overall results showed no performance benefit from napping. But a closer look found that the runners who improved with a nap had averaged 6.4 hours of sleep the night before, while those who saw no improvement had averaged 7.5 hours.

“It seems likely that a nap is most effective when an athlete encounters some form of sleep disruption the night before exercise,” Blanchfield says, “or in those sports where early morning training is standard practice – for instance, swimming.”

The duration of the nap also matters. In the Tunisian study, the 90-minute nap produced better results in the sprint test and had more antioxidant effects. The 20-minute nap, on the other hand, led to better scores in the reaction time and decision-making test, perhaps because waking up from a deeper sleep leaves you feeling groggy for longer.

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That’s why Charles Samuels, the head of the Centre for Sleep and Human Performance in Calgary, typically recommends limiting naps to between 15 and 30 minutes unless you’re trying to make up for an acute bout of missed sleep or travel fatigue. At a conference for Canadian Olympic team staff last fall, he suggested having a coffee before starting this kind of ultrabrief nap, to help kick-start your alertness when you wake up.

And if the logistics of such a short nap seem daunting, Samuels had some reassuring words. “When I say nap, I mean lying down, eyes closed, deep breathing,” he said. “I couldn’t care less if you sleep. This is about brain rest.”

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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