Researchers at UCLA's Neruopsychiatric Institute worked with colleagues at the Veterans Affairs' Neurobiology Research Laboratory.
The findings suggest the cessation of activity in histamine cells causes loss of consciousness during sleep. Similarly, cessation of activity in other brain cells -- those containing the chemicals norepinephrine or serotonin -- causes loss of muscle tone during sleep.
The findings also help explain why anti-histamines, often taken to control allergies, cause drowsiness, the scientists said.
"Our findings greatly improve our understanding of the brain activity responsible for maintaining consciousness and muscle tone while awake," said Dr. Jerome Siegel, senior author of the study. "The findings should aid in the development of drugs to induce sleep and to increase alertness."