An international team of biologists has successfully identified some of the brain chemicals that may help clarify some unanswered questions about how humans sleep.

The research - conducted by the University of California, Los Angeles (UCLA) and the University of Toronto - focused on seals and the chemicals found in their brain, as they are able to sleep with half their brain at a time.

Professor John Peever of the University of Toronto said: "Seals do something biologically amazing - they sleep with half their brain at a time. The left side of their brain can sleep while the right side stays awake. 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."

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The study's first author, PhD student Jennifer Lapierre, measured how the brain chemicals change while the seals are asleep and awake. She found that acetylcholine - an important brain chemical - was at low levels on the sleeping side of the brain, but high levels on the waking side. This discovery suggests that acetylcholine may be responsible for brain alertness.

They also discovered - to their surprise - that the chemical serotonin was present in both sides of the brain whether the seal was awake or asleep. It was previously thought that serotonin caused brain arousal.

Researchers hope that the discovery of the chemicals may make a breakthrough in understanding and curing sleeping disorders. The study's senior author, Jerome Siegel from UCLA's Brain Research Institute added: "Understanding which brain chemicals function to keep us awake or asleep is a major scientific advance. It could help solve the mystery of how and why we sleep."