Stage 1: When you first lie down, you slip into this brief transitional sleep. You might experience involuntary muscle spasms, and if someone says your name, you'll wake up.

Stage 2: You spend about half of your sleep in this stage, during which body temperature drops and breathing and sensory perception slow down. Research shows that this stage may help solidify neural connections related to skills that require motor memory, like riding a bike. Twenty-minute power naps consist mostly of Stage 2 sleep.

Slow-Wave Sleep: During this deep slumber, breathing becomes slower and body temperature drops further. You're unlikely to wake up if someone says your name, though you might go for a walk—most sleepwalking takes place during slow-wave sleep. If you're overtired, your body will prioritize slow-wave sleep, pushing the other stages to the side.

REM Sleep: The rapid-eye-movement stage, during which you dream, typically takes place at the end of each sleep cycle. Brain activity becomes more intense and breathing and heart rate speed up. The body loses the ability to regulate temperature, so you don't sweat or shiver. Researchers think REM sleep is associated with complex learning (like mastering the intricacies of a new language). Also, your muscles are temporarily paralyzed during REM, preventing you from acting out your wildest dreams—usually a good thing.

A typical 90-minute sleep cycle: Stage 1, Stage 2, slow-wave, Stage 2, REM, Stage 2, slow-wave, Stage 2, REM