Our bodies have an internal clock which tells us when to sleep and when to wake up.
The scientific name for the function that controls our sleep cycle is called the circadian clock.
This internal mechanism accomplishes its timekeeping function by monitoring of body temperature, its hormones levels and your general level of alertness.
There is a cluster of about 3000 neurons behind your eyeballs that scientists have named the suprachiasmatic nucleus (SCN).
This cluster plays a significant role in your body's timekeeping function.
These neurons pulse and send rhythmic signals to the pituitary gland which then send chemicals into the blood.
It also sends these same chemicals to the pineal gland which in turn releases melatonin.
Studies have found that it is the build up of melatonin that causes us to become drowsy.
There is another neurotransmitter, a chemical called adenosine that is produced while we are awake.
Your cells produce power to get you through the tasks you need to do each day and adenosine is a bi-product of the use of that power.
At the end of the day, adenosine builds up to a level that tells our brains its time to sleep.
The longer we resists the more adenosine and melatonin build up and we feel sleepier and sleepier until finally we fall asleep.
While we sleep these hormones are broken down by other bodily functions, our circadian clock is reset and the cycle starts all over again.
Stages or Levels of Sleep Let's take a quick look at what happens as we sleep.
Right before we go off into dreamland we spend a little time in a state known as "relaxed wakefulness".
This is experienced by that "not quite awake" and "not quite asleep" feeling that we all experiences right before we fall asleep or right when we begin to wake up.
This state usually lasts somewhere between two and ten minutes and then you "fall asleep".
You are considered to be really sleeping when you're posture is relaxed.
For most people this is when you are sitting or lying down.
There is decreased movement of your skeletal muscles.
You also experience a diminished response to external stimulation such as sounds and touch and your metabolism slows down.
Scientists have divided sleep into two categories.
The first is REM sleep which is short for Rapid Eye Movement.
The other category is know as NREM or non-REM sleep.
You go through four stages of NREM sleep.
The first stage or Stages I is called drowsiness.
This is where your eyes move slowly and your body adjusts in the bed or the chair.
Stage I usually lasts about ten minutes.
You enter Stage II or light sleep when your eye movement stops, your heart rate slows down and your body temperature drops slightly.
The remaining two stages, Stages III and IV are known as deep sleep.
In deep sleep you have no eye movement or muscle activity.
People in deep sleep are hard to wake.
People awakened out of deep sleep feel disoriented, confused and groggy and are sometimes known to act irrationally for a short while after they are awakened.
That leaves us with REM sleep.
This is when we dream.
Studies show that brain activity increases to nearly the same level as a person that is awake during REM sleep.
Somewhere between 1 to 1 ½ hours after you fall asleep you enter REM sleep.
A REM period will last only a few minutes.
You'll breathe irregularly and more rapidly.
Your heart rate and blood pressure rise.
Men often experience erections.
During REM your eyes jerk around rapidly in many directions under your eyelids.
This movement is what gives REM sleep its name.
Most people have three to five REM periods a night.
So that's how your body knows when to sleep and the levels you experience while you do sleep.
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