New York, March 8 (IANS): Melatonin supplements are commonly taken as a sleep aid but new research reveals that even in the absence of a supplement, naturally occurring melatonin hormone may help us fall asleep.

"The study on zebrafish suggests that the melatonin naturally made in our brains may also be important for sleep," said David Prober, assistant professor of biology from California Institute of Technology.

For years, researchers have known that melatonin production is regulated by the circadian clock and that animals produce more of the hormone at night than they do during the day.

In the hopes of determining, once and for all, the role that hormone actually plays in sleep, Prober and his team designed an experiment using the larvae of zebrafish that has a well-characterised genome.

Like humans, zebrafish are also diurnal - awake during the day and asleep at night - and produce melatonin at night.

But how exactly can you tell if a young zebrafish has fallen asleep?

There are behavioural criteria - including how long a zebrafish takes to respond to a stimulus, like a knock on the tank, for example.

"Based on these criteria, we found that if the zebrafish larvae do not move for one or more minutes, they are in a sleep-like state," Prober noted.

To test the effect of naturally occurring melatonin on sleep, the researchers first compared the sleep patterns of normal zebrafish larvae to those of zebrafish larvae that are unable to produce the hormone.

They found that fish with the mutation slept only half as long as normal fish.

And although a normal zebrafish begins to fall asleep about 10 minutes after "lights out" - about the same amount of time it takes a human to fall asleep - it took the mutant fish about twice as long.

"This result was surprising because it suggests that almost half of the sleep that the larvae are getting at night is due to the effects of melatonin," Prober pointed out.

That suggests that melatonin normally plays an important role in sleep and that you need this natural melatonin both to fall asleep and to stay asleep, the authors noted.

In both humans and zebrafish, melatonin is produced in a part of the brain called the pineal gland.

The study was published in the journal Neuron.