Sleep researcher Nicole Lovato writes about the benefits of afternoon naps (something I’m a big believer in):

It’s common, occurs whether you’ve eaten lunch or not, and is caused by a natural dip in alertness from about 1 to 3pm. So, if you find yourself fighting off sleep in the middle of the day and you’re somewhere where you can have a nap, then do it.

Taking the time for a brief nap will relieve the sleepiness almost immediately and improve alertness for several hours after waking. And there are many other benefits too.