One of our body's natural hormones,
melatonin, which is produced by our brains during the night, helps
us to regulate sleep. However the pace of today's modern world,
which leaves our brains in a constant state of overdrive, means
that more and more of us are unable to produce this hormone
naturally and are turning to the supplement version of
melatonin. 


Our understanding of melatonin is
very much from the human perspective but where did it all begin?
According to a study by the University of Heidelberg in Germany,
animals have used melatonin in evolutionary development for years,
going as far back as marine animal life, zooplankton.


Zooplankton's routine which sees
them reach the surface of the water at dusk and dive back down at
night, is their deep-rooted biological mechanism and may also go
some way in explaining our own circadian rhythms. Light-sensitive
cells found in the zooplankton brain were very closely matched with
those found in the human pineal gland - the cells which produce
melatonin and help to control our circadian rhythm.


Although the zooplankton's routine
isn't exactly the same as sleep as we know it, the researchers were
convinced that this is the closest thing they have found in simple
organisms which emulates human sleep.


Source: 
https://www.sciencenews.org/blog/scicurious/melatonin-and-watery-beginnings-sleep