Melatonin (N-acetyl-5-methoxytryptamine) is an evolutionary highly con
served molecule that plays an important role in conveying the clock an
d calendar information to all living organisms, including man. Melaton
in is synthesized in the rhythmic fashion, primarily by the pineal gla
nd, and, to a lesser degree, by extrapineal tissues - namely the retin
a, the Harderian gland, and the gastrointestinal tract. The rhythm of
the hormone production, with maximal levels occurring at night in dark
ness, is generated by an endogenous circadian clock(s) and is synchron
ized with the photoperiodic environment to which animals are exposed.
This brief outline surveys data on the regulation of rhythmic melatoni
n biosynthesis by a circadian pacemaker and light (full spectrum white
light and monochromatic lights with wavelengths both in the visible a
nd invisible range). Additionally, possible applications of this chron
obiotic compound in agriculture and in medicine in the treatment of ci
rcadian rhythm sleep disorders are discussed.