Insomniac brains are both asleep and awake.

Have you ever gotten out of bed still tired because you'd been worrying all night? New research suggests that your brain might have been both asleep and awake through those fitful hours.

Stress-induced insomnia affects almost a quarter of people in the United States each year. Yet, scientists know little about what the brain's doing as people struggle to get some shut-eye.

To investigate, Clif Saper and Georgina Cano of Harvard University and their colleagues induced anxiety in drowsy rats by letting them fall asleep in a clean cage and then transferring them to a dirty cage previously occupied by another rat.

Because rats are territorial, says Cano, the transferred animals became anxious and were unable to immediately settle back into sleep. When they did rest, the rodents slept fitfully, waking frequently.

After observing the animals for several hours, the team examined the rats' brains. The researchers found that brain areas such as the cortex, which are normally turned on only during waking hours, were active during the fitful sleep. Also active were brain areas such as the ventral lateral preoptic area, which typically turn on only during sleep.

"It was a surprising finding because the animals' brains appeared neither truly awake nor truly asleep," says Cano.

She suggests that these results could explain why some people in sleep clinics report being awake all night, even though their brain waves indicate that they slept. The findings might also guide researchers to drugs that could promote sleep by turning off select parts of the brain rather than the more-widespread neural areas that existing sleeping pills shut down.--C.B