Health Sachet No 2:

Sleep is not just a simple snooze. It has many components that play different roles in our health.

(Added in 2013: Here is the detailed description of the Sleep Architecture, or composition:

Sleep is composed of two stages: REM (Rapid Eye Movement) and non-REM. The former helps in mental consolidation while the latter helps in physical repair and rebuilding. During the night, you alternate between REM and non-REM stages 4-5 times.

The earlier part of sleep is mostly non-REM. During that period, your pituitary gland releases growth hormones that repair your body. The latter part of sleep is more and more REM type.

For you to be mentally alert during the day, the latter part of sleep is more important. When you wake up with an alarm clock after 5-6 hours of sleep, you are mentally irritable throughout the day (lack of REM sleep).

On the other hand, if you have slept for less than 5 hours, your body is in a physical mess (lack of non-REM sleep): you are tired throughout the day, moving like a zombie and your immunity is way down.

If you want to know if you are getting adequate sleep, take Epworth Sleepiness Test: