The Wikipedia page for Circadian Rythm gives you a quick overview about this subject, with lots of links for further reading.
A circadian rhythm is an endogenously
driven roughly 24-hour cycle in
biochemical, physiological, or
behavioural processes. Circadian
rhythms have been widely observed, in
plants, animals, fungi and
cyanobacteria.
...
A great deal of research on biological
clocks was done in the latter half of
the 20th century. It is now known that
the molecular circadian clock can
function within a single cell; i.e.,
it is cell-autonomous. At the same
time, different cells may communicate
with each other resulting in a
synchronised output of electrical
signaling.
...
Circadian rhythmicity is present in
the sleeping and feeding patterns of
animals, including human beings. There
are also clear patterns of core body
temperature, brain wave activity,
hormone production, cell regeneration
and other biological activities.
...
The primary circadian "clock" in
mammals is located in the
suprachiasmatic nucleus (or nuclei)
(SCN), a pair of distinct groups of
cells located in the hypothalamus.
Destruction of the SCN results in the
complete absence of a regular
sleep–wake rhythm.
...
Timing of medical treatment in
coordination with the body clock may
significantly increase efficacy and
reduce drug toxicity or adverse
reactions [...] There are many health problems associated with disturbances of the human circadian rhythm.
...
Although circadian rhythms are
endogenous ("built-in",
self-sustained), they are adjusted
(entrained) to the environment by
external cues called zeitgebers, the
primary one of which is daylight.
