Using the imaging technique positron emission tomography, 15 healthy participants' brains were tested after being kept awake all night or after being allowed to sleep.

Compared to when well-rested, participants when sleep deprived showed reduced binding of a radiolabeled compound that binds to dopamine receptors in certain parts of the brains.

The researchers concluded that sleep deprivation increases dopamine -- in the striatum part of the brain -- involved in motivation and reward -- and in the thalamus -- involved in alertness. Following sleep deprivation, the rise in dopamine, the researchers say, may promote wakefulness to compensate for sleep loss.

"However, the concurrent decline in cognitive performance, which is associated with the dopamine increases, suggests that the adaptation is not sufficient to overcome the cognitive deterioration induced by sleep deprivation and may even contribute to it," study leader Dr. Nora Volkow, of the National Institute on Drug Abuse, National Institutes of Health in Bethesda, Md., said in a statement.

The study findings are published in The Journal of Neuroscience.