When night falls and you go to sleep, time seems to stop. When we slide into unconsciousness, we enter an uncanny state of being where we don't consciously monitor what's going on in our bodies or our minds. What really happens after you fall asleep at night, on this Moment of Science.

Sleep is divided into distinct cycles that contain four different types, or stages, of sleep. You experience stage one when you doze off at the office in the late afternoon, but you're still aware that you're sitting in front of your computer. Stage one sleep is characterized by light dozing, barely over the brink of wakefulness.

But if you're headed toward some serious snoozes, stage one only lasts a couple of minutes before it changes into stage two sleep. Stage two sleep is the transitional phase where you'll spend about half of your total slumber time. After about fifteen to twenty minutes of stage two sleep, you'll move into stage three and then stage four sleep. Because the activity in your brain slows to a snail's pace during these stages, sleep researchers refer to it as "slow wave sleep." As you get older, you'll spend less time in slow wave sleep.

Slow-wave sleep usually lasts about forty minutes, completing the first sleep cycle. At the end of this cycle, you'll roll over and wake up for fifteen to twenty seconds before falling back into stage two sleep. This second round of stage two sleep usually precedes a stage called rapid eye movement, or REM sleep. Dreams occur only during this separate stage of sleep.