Serotonin and Melatonin are two hormones made from
the amino acid Tryptophan.  Serotonin is made directly
from Tryptophan, and Melatonin is made from Serotonin.   
Both Serotonin and Melatonin are secreted by a
pea-sized, pine-cone-shaped gland called the pineal
gland.  The pineal gland is located in the center of the
brain.  Instead of secreting the Serotonin and Melatonin
directly into the blood, the pineal gland secrets those
hormones primarily into the small, inner serial/spinal fluid
holding verticals and channels inside the brain,
cerebellum and spinal cord.  Most of the Melatonin
receptors in the body are located in the inside parts of the
central nervous system that line the verticals and canals
that contain the ceribrial/spinal fluid.  So the
ceribial/spinal fluid is what dissolves and carries
Serotonin and Melatonin to their receptors.  

Since there are normally only a few ounces of
ceribrial/spinal fluid in the entire central nervous system,
while the rest of the body usually weighs hundreds of
times more than the ceribrial/spinal fluid, secreting
Serotonin and Melatonin directly into the fluid of the brain,
rather than into the blood, greatly reduces how much
Serotonin and Melatonin must be secreted to adequately
stimulate those receptors.    That is also why humans
need to take relatively huge (20 - 200 mg) oral doses of
supplemental Melatonin to directly and fully stimulate the
Melatonin receptors that line the ventricals and canals of
their brains.  

Yet supplemental doses of just 1/3 - 3 mg of Melatonin
are commonly used to help people with insomnia to fall  
asleep.  The are two reason why so little, taken orally and
diluted by all of the fluid in the entire body, can still help
to put middle-aged people to sleep.  The first reason is
that as we age our pineal glands are programed to
produce less and less Melatonin.  A young child's pineal
gland typically produces about 20 times as much
Melatonin as their father's pineal gland, and if their
grandfather is still alive, about 100 times more Melatonin
than their grandfather's pineal gland.   The reason
middle-aged people can still usually sleep normally,
despite their declining Melatonin production, is because
with their lower Melatonin levels, fewer of their Melatonin
receptors produced inside their central nervous systems
get used and consumed.  Thus, as their Melatonin
production declines, they accumulate more and more
Melatonin receptors, and thereby become more and more
sensitive to Melatonin.
The second reason that small doses of Melatonin can
help induce sleep is because they don't act alone.   
Rather, a slightly rising Melatonin level, as cause by
taking a low dosage Melatonin pill, triggers the pineal
gland to suddenly increase it own Melatonin secretion.
This probably evolved so that once part of the pineal
starts its Melatonin production, then the rest of the gland
can also switch to creating Melatonin from Serotonin.  
This helps to speed the  raising of the Melatonin level
and as made from the Serotonin that builds up in the
cerebrial spinal fluid while we are awake, it also lowers
Serotonin level there.  The rapid rising of the Melatonin
level and lowing of the Serotonin level in the cerebrial
/spinal fluid helps to quickly switch the central nervous
system from waking mode, into sleeping mode.   
These phenomenon are also why people who have
suffered from insomnia, and are just starting taking
Melatonin, to teat their insomnia, often end up feeling
groggy the next morning, even though they may be taking
relatively small doses of Melatonin.   They often start off
very sensitive to Melatonin, so the sudden increase in
their own production, that can be triggered by the small
supplemental dose, may be enough to help them sleep.  
Even so, it may not stimulate enough extra Melatonin
production to consume that many of their high proportion
of Melatonin receptors.   Thus, the next morning, they are
likely to still have enough Melatonin receptors left over to
be relatively sensitive to Melatonin, and they may also
still have high enough residual Melatonin levels to
stimulate enough of their residual high level of Melatonin
receptors into keeping them partly in sleeping mode.   
After taking supplemental Melatonin for about a week,
however, the excess Melatonin receptors are consumed,
so higher levels of Melatonin can be enjoyed without
producing grogginess the next morning.

While Melatonin helps put us to sleep, it is not what
usually keeps us asleep during the later phases of sleep.  
Rather, a very common natural steroid hormone called
DHEA (DiHydroEpiAndosterone) is what keeps us
sleeping during the later phases of normal sleep.  DHEA is
also what drives most of the intensified dreaming,
memory consolidation, bodily repair and hormonal
recharging that peak during the later hours of sleep, even
while Melatonin production, and levels, are declining and

At some point as we age our Melatonin production
eventually drops too low to stimulate enough Prolactin
and DHEA production to enable the later phases of sleep
that higher levels of DHEA drive.  Reduced DHEA
production due to slowing metabolism and/or adreanal
gland decline can also undermine our nightly production
and levels of DHEA, so that there is too little DHEA to
support the later phases of sleep and dreaming.  This
results in the insomnia that commonly develops in the
elderly, and/or the stressed, during the middle of the
night.  Such insomnia often presists for hours.  I believe it
occurs because, instead of driving sleeping and dreaming
like high DHEA levels do, moderate DHEA are stimulatory
of day time levels of conousness.  With insomnia during
the middle of the night, DHEA production and storage are
suppressed, so that during the middle and later parts of
the next day, the body's store of DHEA from the previous
night usually end up too small and too quickly consumed
to support normal waking consciousness.  Instead, such
insomnia victims usually feel very tired the next afternoon.

Low levels of DHEA produce sensations of tiredness, and
also drive the need to take naps in the middle of the day.  
The best time to have low DHEA levels is at the end of
the day, when they will make you feel tired and make you
to want to go to bed early and get a good nights sleep.  

The lreason large concentrations of Melatonin normally
occur in the cerebrial-spinal fluid of young and healthy
people during the early phases of sleep is to counteract
the stimulatory effects of the rising and still only
moderately high levels of DHEA that are secreted during
the early hours of normal sleep.   What
supplementary doses of Melatonin do to counter act the
Melatonin insuficency in those who have sleeping
problems is not so much to directly raise the Melatonin
level very much, as to provide the peneal gland with just
enough Melatonin to trigger it to start producing more
Melatonin on its own.  The pineal gland has evolved to be
sensitive to its own Melatonin production.  That way it
can switch more quickly from producing and secreted the
Serotonin that drives and maintains our brains in waking
modes of though, to converting the Serotonin that has
already been produced and accumulated, into Melatonin,
which is then secreted to drive our sleep, Prolactin and
DHEA production, and the more general hormonal
recharging that normally occurs while we are sleeping.   

With a small amount of supplemental Melatonin to trigger
then pineal gland to more quickly and intensively switch
to consuming Serotoinin from which to make and secret
Melatonin, even with the greatly decreased overall
Serotonin and Melatonin production associated with
aging, our brains can still switch fairly quickly into
sleeping mode.  Then, among other things, the higher
brain Melatonin levels of sleeping mode drive our pituitary
glands to secrete more Prolactin, which then drives our
bodies to produce more DHEA (DiHydroEpiAndosterone).
The increased Melatonin also helps to put us in sleeping
and dreaming mode, and also drives our brains consume
more DHEA.  The DHEA is used to recharge our daytime
reserves of DHEA, and also to drive the increased repair,
dreaming and long-term memory consolidations
operations of our brains that occur while we are

Melatonin also switches off the connections of the
dreaming brain from the rest of our bodies.  That way,
while we are dreaming, we do not end up talking,
accidentally battering our mates, sleep walking, and/or
trying to drive our cars, while we are dreaming about
talking, fighting, walking and/or driving.  If your mate tells
you that you talk or walk in your sleep, that may be a sign
of low Melatonin levels.  Our Melatonin levels normally
pick during the early hours of sleep, and drop to almost
nothing during the later hours of sleep.