What exactly is a good night's sleep?

The evidence is clear that a good night's sleep (night after night, not once every couple of weeks) is fundamental to both your health and your longevity. But what exactly is a good night's sleep? While the length of sleep is important, equally vital is getting through the sleep cycle several times. The cycle comprises the following stages:

1. Sleep latency: This is the time it takes for you to fall asleep from the time you go to bed.
2. Stages 1 and 2: light sleep. Drowsiness as your brain is just getting into sleep. In stage 2, your brain waves start to slow down noticeably, resting those parts you use while awake.
3. Stages 3 and 4: deeper sleep, which you get less of as you age because of frequent awakenings. These stages are both restorative. Both REM sleep and sleep stages 3 and 4 are homeostatically driven—that is, if a human is selectively deprived of one of these, it rebounds once the person is allowed to sleep. This suggests that both are essential in the sleep process and its many functions. REM (rapid eye movement): the deepest sleep. Your eyes are moving fast, but the rest of your body is paralyzed. It's the stage where some sleep-related disorders take place, like sleepwalking.

Each one of the sleep cycles lasts about 90 minutes, and you go through four to six of them a night. But the important part is that you have to get to REM sleep to feel rested. People who have sleep problems often don't make it to REM sleep because it takes up to 60 minutes to make it to REM. If you're having frequent awakenings before you make it to REM, then you're never getting that restorative, healthy sleep.