The sleep habits of seals are helping researchers learn more about what happens in the human brain when people have trouble staying asleep.
"Seals do something biologically amazing -- they sleep with half their brain at a time," said Professor John Peever of the University of Toronto. "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" -- with both parts of their brain at once. "Our research may explain how this unique biological phenomenon happens."
An international team of scientists from the University of California, Los Angeles (UCLA) and the University of Toronto observed the chemical brain cues that make this possible, with results that may help explain how the human brain stays alert during waking hours and winds down during sleep.
The study was published this month in the Journal of Neuroscience after years of observing northern fur seals.
Jennifer Lapierre, the University of Toronto PhD student who led the study, discovered this phenomenon by measuring how different chemicals change in the sleeping and waking sides of the brain in seals.
Previous research showed that acetylcholine (Ach), an important brain chemical, was at low levels on the sleeping side of the brain but at high levels on the waking side -- suggesting that Ach drives brain alertness on the side that is awake.
However, the team also made the surprising discovery that another major brain chemical, serotonin, was present at the same level on both sides of the brain whether the seals were awake or asleep.
This contradicts the prevailing theory among scientists that serotonin is a chemical that causes brain arousal.
Even when the seals were stimulated by eating and being hosed with water, serotonin levels spiked on both sides of the brain. This suggests that serotonin is not necessarily linked to wakefulness, but to motor and autonomic responses that involve both sides of the brain.
Dr. Peever told the Canadian Press that he believes seals' underwater sleep behavior evolved to help them ward off prey that attack from underwater.
"The side of their brain that's looking down into the water is ... the side that's watching, and the eye that's looking up into the sky where there's no harm coming from typically is the side that is sleeping."
Other aquatic mammals like whales and dolphins have the same half-sleep behavior. The scientists behind this study believe that seals are especially useful to compare to humans because they are smaller and can be observed on land.
The research team believes the study has important implications for understanding and treating human sleep disorders, with the difference between the sleeping and waking parts of the seal brain shedding light on the difference between the sleeping and waking states of the human brain.
According to senior author Jerome Siegel of UCLA's Brain Research Institute, "about 40 percent of North Americans suffer from sleep problems and 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."