Going without sleep for an extended period clearly impairs your ability to learn, respond quickly, and perform complex tasks in the face of distractions or changing information. Some research shows similar effects from getting even an hour or two less than the optimal seven to eight hours of sleep each night.

And the effects are cumulative. In one study, people who slept for six hours a night for two weeks performed as poorly on cognitive tests as those who stayed up for 24 hours. That holds true even if you don't feel particularly sleepy. "People can shortchange themselves night after night, and after awhile, their sense of sleepiness tends to level out," says Jeffrey Durmer, M.D., Ph.D., an adjunct professor of allied health sciences at Georgia State University. "But we still find observable effects on their cognitive ability."

Practice good sleep hygiene by going to bed and waking at the same time each day, avoiding beverages (especially alcoholic or caffeinated ones) a few hours before bedtime, and exercising early in the day. And as long as it doesn't interfere with your nighttime sleep, a short nap can be a real brain refresher.