You feel completely wiped out, but you know you need to push through for just a little bit longer. Do you reach for a mug or a pillow? Here's what science has to say.
When to Take a Nap
Overall, naps have a lot of advantages over drinking coffee. They can boost your brain's learning capacity, improve alertness, and generally improve your brain's cognitive abilities. Plus, naps don't have a crash associated with them like caffeine does. In a landmark study conducted by the Department of Psychology at the University of California, San Diego, napping for 60 to 90 minutes was found to improve performance in perceptual learning, procedural motor skill, and verbal memory considerably more than caffeine.
The downside to napping is that, well, it takes 60 to 90 minutes, and you might not have that extra time lying around. According to a study from Sara Mednick, Ph.D., in the journal Nature Neuroscience, a power nap that lasts 10 to 30 minutes will probably still get you further than a cup of joe, but it has to be timed just right. A nap that goes beyond 30 minutes, but doesn't make a full sleep cycle of 60 to 90 minutes, will likely cause a decline in your motor dexterity and increase your grogginess due to sleep inertia. You experience sleep inertia every morning when you wake up (so you know what this grogginess feels like), and it can take several hours for it to dissipate. You also don't want to nap too late in the day or you'll end up throwing off your circadian rhythm. That can throw off your sleep for the whole night, making you even more tired tomorrow.
Make napping your number one choice if you have the time, but aim for around 20 minutes of actual sleep (giving yourself a 30 minute window helps), and plan to nap sometime between 1:00 and 3:00 p.m when your brain is most tired.
When to Drink Coffee
If you can't nap, coffee is a decent alternative. It doesn't take up as much time as napping would. and you don't have to be around a bed or sleep pod in order to actually fall asleep. It's definitely the easier choice, no matter how you look at it.
Caffeine has its own advantages, too. It will probably make you feel more physically awake than a nap, for example. In the University of California, San Diego study mentioned earlier, participants felt physically better when testing under the effects of caffeine compared to those who napped. Their test scores were lower, but they didn't feel groggy and miserable like the napping group. Caffeine may also be a better choice for middle-aged people who want to stay alert. In a French study, published in the journal Sleep, participants aged 40 to 50 years drove a vehicle on a course better than those in the same age group who napped. For younger participants in the study, napping was still more beneficial, but for older folks a little coffee might do more good than a nap. Regardless of how old you are, however, an Australian study suggests that driving under the effects of caffeine when you're groggy is almost always better than nothing. (But not driving at all when you're groggy is certainly the safest option.)
In short: good for making you feel more physically awake, keeping you attentive while you drive, and is one of the only options when you're crunched for time. But in general, coffee is should be your second choice to napping. Studies have also shown that caffeine can improve physical performance in cyclists and improve overall physical endurance in athletes.
When You Can, Do Both
Ah, the fabled "coffee nap." The concept originated from multiple studies that came out of Loughborough University in the UK. You drink a cup of coffee, then fall asleep, and when you wake up the caffeine will just be starting to kick in. A study led by Mitsuo Hayashi, and published in the journal Clinical Neurophysiology, found a coffee nap to be more effective than using bright lights or face washing to become more alert post-nap. Another study, published in the journal Ergonomics, also suggested coffee naps were more effective in maintaining nocturnal alertness and performance than regular naps.
Coffee naps, or caffeine naps, are your best bet overall if you can swing them. The catch, though, is you need to fall asleep before the caffeine kicks in. If you can manage that, this method gives you the benefits of napping, minus the grogginess associated with waking up. Worst case scenario, you at least get some benefits from the caffeine. When it's all said and done, nothing is better than getting a good night's sleep, but when you need to get by, you at least have a few tools under your belt.