If you use an alarm clock to wake up, it’s a good bet you’re sleep deprived. But what if you feel fine when that buzzer, bell, or blast of music rouses you? You’re probably still not getting all the sleep you need — the fundamental definition of sleep deprivation — or you wouldn’t have required the jolt. You would have woken up by yourself already.

“You can keep eating when you’re not hungry, but you can’t keep sleeping if you’re not tired,” says Dr. Elizabeth Klerman, a professor of neurology at Massachusetts General Hospital and the Division of Sleep Medicine at Harvard Medical School.