What makes us sleep ?

Although you may put off going to sleep in order to squeeze more
activities into your day, eventually your need for sleep becomes
overwhelming and you are forced to get some sleep. This daily
drive for sleep appears to be due, in part, to a compound known as
adenosine. This natural chemical builds up in your blood as time
awake increases. While you sleep, your body breaks down the
adenosine. Thus, this molecule may be what your body uses to
keep track of lost sleep and to trigger sleep when needed. An
accumulation of adenosine and other factors might explain why,
after several nights of less than optimal amounts of sleep, you build
up a sleep debt that you must make up by sleeping longer than
normal. Because of such built-in molecular feedback, you can’t
adapt to getting less sleep than your body needs. Eventually, a lack
of sleep catches up with you.
The time of day when you feel sleepy and go to sleep is also
governed by your internal “biological clock” and environmental
cues—the most important being light and darkness. Your biological
clock is actually a tiny bundle of cells in your brain that responds to
light signals received through your eyes. When darkness falls, the
biological clock triggers the production of the hormone melatonin.
This hormone makes you feel drowsy as it continues to increase
during the night. Because of your biological clock, you naturally
feel the most sleepy between midnight and 7 a.m. You may also feel
a second and milder daily “low” in the midafternoon between
1 p.m. and 4 p.m. At that time, another rise occurs in melatonin
production and might make you feel sleepy.