Scientists have identified brain chemicals that allow seals to sleep with half of their brain at a time, according to a study published in the Journal of Neuroscience. And they say the discovery of how what they called a \"unique biological phenomenon\" works may help millions -- including an estimated 40 percent of North Americans -- who suffer from insomnia or other sleep disorders. \"It could help solve the mystery of how and why we sleep,\" said the study\'s senior author Jerome Siegel of UCLA\'s Brain Research Institute. Co-author John Peever of the University of Toronto explained that seals are able to \"something biologically amazing... The left side of their brain can sleep while the right side stays awake.\" The animals sleep this way while they\'re in water, but they sleep like humans while on land, he said. Scientists measured how chemicals change in the sleeping and waking sides of a seal\'s brain, using surface scanners to measure brain electrical activity and tubes inserted in the cortex to measure chemicals. Acetylcholine was found at low levels on the sleeping side of the brain but at high levels on the waking side, suggesting that it may drive brain alertness on the side that is awake. Another important brain chemical -- serotonin -- was present in equal levels on both sides of the brain at all times. This is surprising, said Peever, because scientists long thought serotonin was a chemical that causes brain arousal.