Scientists are learning more about how sleep works by studying seals.

Researchers from the University of Toronto and the University of California say that fur seals sleep with half their brain at a time. "The left side of their brain can sleep while the right side stays awake," said Professor John Peever, in a news release from the University of Toronto.

"Seals sleep this way while they’re in water, but they sleep like humans while on land. Our research may explain how this unique biological phenomenon happens."

The researchers are studying the chemicals active in seals' brains to help solve the mystery of how and why humans sleep.

Their study, published in this month's Journal of Neuroscience identified chemical cues that allow the seal brain to remain half awake and asleep. The findings may explain the biological mechanisms that enable the brain to remain alert during waking hours and go off-line during sleep.

The study’s first author, University of Toronto PhD student Jennifer Lapierre found that acetylcholine – an important brain chemical – was at low levels on the sleeping side of the brain but at high levels on the waking side. But the study also showed that another important brain chemical – serotonin – was present at the equal levels on both sides of the brain whether the seals were awake or asleep.

It is estimated that about 40 per cent of North Americans suffer from sleep problems and understanding brain chemicals "could help solve the mystery of how and why we sleep" said Jerome Siegel of UCLA’s Brain Research Institute.