Usually,
when we go to sleep, it is simply because we are tired or want to have energy
when we wake up. To get to sleep, most people lie down on their bed, close their
eyes and fall asleep. Perhaps at times a yawn seems to be the first indicator
of sleepiness.
What we are looking at here is how one changes from the state of being awake to being completely asleep. Scientists have just recently (in the past few decades) started studying exactly how sleep is different from being awake.
Sleep is not an abrupt change from wakefulness. To become completely asleep you proceed through gradual steps. Each of these steps has a particular pattern and follows its Circadian Rhythm. A normal night's sleep would start off by the thoughts of the person becoming hazy and reacting less to external occurrences. The muscles in the body become relaxed. Body temperature, heart rate and blood pressure slowly drop.
As the person passes from wakefulness and enters the NonREM sleep, the level of serotonin in the brain increases. This process is basically how the body makes the transition from being awake to asleep.
Once the person is fully asleep, they have pretty much reached stage 2.
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