Wake up, sleepy brain.

Lying with one's head immobilized under a brain scanner and an intravenous tube in one's arm is not a pleasant way to doze off, but seven men did just that in the name of science. Their ability to snooze in the face of such indignities has yielded a view of what happens in the brain during dreams.

The men, deprived of sleep in advance so they would slumber deeply, underwent positron emission tomography (PET) scans while awake, shortly after falling asleep, and during the sleep phase characterized by rapid eye movements and the occurrence of dreams. Volunteers were awakened while still in the dreaming phase, and each could recall his dreams.

During dream sleep, blood flow rose sharply in several brain areas, particularly the thalamus, assert Pierre Maquet of the University of Liege in Belgium and his coworkers. The thalamus, already linked to the processing of memories of emotional experiences, may reactivate emotional components of memories during dreaming, the scientists suggest in the Sept. 12 Nature.

Dreamers also displayed declines in blood flow in complex thinking areas at the front of the brain. Sparse neural activity there may contribute to a distorted sense of time and loss of self-awareness during dreaming, as well as to the forgetting of dreams upon awakening, the scientists theorize.