The first thing I want to say about naps is that I’m almost always for them. Naps can help you recover from sleep deprivation. Naps are good.
But let’s talk about that almost always. When might you want to avoid napping?
Well, maybe you’re trying to shift your personal time zone and are at a point in your internal circadian day where getting light exposure will be very, very helpful to achieving that shift. Closing your eyes to take a nap will block photons from reaching your retina, which means your brain won’t have the photic momentum it needs to push through a shift in your rhythms. Probably not a big setback if the nap is short, but a multi-hour nap at the wrong time could end up slowing down how quickly you adjust.
Or maybe you really, really need to be alert right at the moment when you’d be waking up from a nap. In that case, you might worry about sleep inertia, the phenomenon of general grogginess and impaired performance that can persist for several hours after waking. This, too, might make you want to hold off on a nap.
And then there is the classic “I napped, ergo, I cannot sleep now.” Your transition into sleep is driven both by your circadian clock as well as your accumulated “hunger for sleep” (or sleep drive). Feed that hunger for sleep right before bed, and you might not have enough sleep pressure built up to flip your switch from on to off. This is one of the reasons why avoiding naps in the evening is a common component of cognitive behavioral therapy for insomnia.
So: You’re sleepy. You’ve got other stuff to do today. Do you take a nap? If so, how long?
Answer: You probably want a ten minute nap.
This, like all science boiled down to a single tidbit, is a big ole simplification. It matters what your internal time is (does your body think it’s day or night?) and what your recent sleep/wake history is.
But multiple studies have found that a 10 minute nap during the day improves performance right off the bat, while longer naps mean that you have to sink time into recovering from your nap after you wake up. A 20 or 30 minute nap in these studies was still found to be better than staying awake, but participants could still be shrugging off the effects of sleep inertia more than two hours after waking, while a 5 minute nap was generally not enough for much of an effect.
Would you ever want to take a longer nap? You might, if your goal is not so much “perform better for the next two hours” as it is “don’t fall asleep in the next ten hours.” In a classic study from 1986, researchers kept subjects up all night, let them take a morning nap, and then measured how readily they fell asleep at different points over the rest of the day. Here, a 15 minute nap was barely better than no nap at slowing down how rapidly people fell back asleep, while a 60 minute nap had alerting effects that persisted 4 to 8 hours later. The benefits didn’t keep increasing past 60 minutes, though: a 120 minute nap didn’t get you anything more than a 60 minute one did.
Another reason to consider a longer nap is memory. People who get a 60 minute nap do a better job at remembering words they’ve been exposed to than people who don’t get a nap. That said, a 6 minute nap is also enough to see a significant memory boost— for one tenth the time investment.
In conclusion: You probably want a short nap. You might want a longer nap, though, if you don’t need to be super alert for the next few hours, but you do need to stay awake later in the day.
Lastly, you might benefit from a nap, even if you don’t think of them as particularly helpful for you. The benefits of napping that show up in objective reaction time tests often aren’t reflected in how people subjectively rate their own sleepiness. You might get more of a boost from naps than you think, and you may also need less of a nap than you’d expect to see that boost.
Much of this blog post was helped along by this review. Thanks to the authors for the great resource!