If you really need one, the best time is between 2 p.m. and 3 p.m. Your blood sugar levels start to dip after lunch, and it’s early enough that it won’t mess with your bedtime. But don’t overdo it -- 20 minutes is enough to make you more alert and put you in a better mood. Push it past 30, and you can hit a deeper stage, when your brain waves slow down. You’ll be groggy when you wake up (and probably grumpy, too).