#01 What is Sleep? (aka What's going to happen when you fall asleep tonight?)

All stages of sleep are critical. Different types of sleep perform different functions for the brain and the body at different times of night. But we need all of those stages of sleep—there is no one stage of sleep that's more important.

In the first half of the night, the majority of your 90-minute sleep cycles [are] going to be comprised of lots of deep non-REM sleep and very little REM sleep. But as you push through to the second half of the night, now that seesaw balance will actually change, and instead, the majority of those 90-minute cycles are going to be comprised of much more rapid eye movement sleep, or dream sleep.

REM sleep is the principal stage within which we dream. Now, that depends on your definition. If you have a loose definition of what dreaming is, well, it turns out that we dream in almost all stages of sleep.

Now, on the other hand, we have rapid eye movement sleep, or REM sleep, named... because of these bizarre, horizontal, shuttling eye movements that occur underneath your eyelids during rapid eye movement sleep.