Many of us look forward to sleep as a respite from the worries of the day. But it is not as free from care as we might hope, according to Jan Born, a neuroendocrinologist at the University of Luebeck in Germany. Even as we doze, our brains somehow keep track of our daily schedule and signal the release of a stress hormone one hour before we have to get up. By then, hitting the snooze button is just fighting the inevitable.

The key to the body’s internal alarm clock is adrenocorticotrophic hormone, or ACTH, which helps regulate the nervous system. In response to a stressful situation--being pursued by a pack of wolves, for instance--or even the anticipation of such an event, the pituitary gland pumps out ACTH, which in turn triggers other hormones that prepare the body to fight or run.

Production of stress hormones is suppressed during sleep, but concentrations of ACTH in the blood increase before a person wakes up. Sleep researchers used to believe this pattern is invariable, a part of the body’s normal, daily (or circadian) rhythms. But Born and four university colleagues wondered if the pituitary gland could respond to an anticipated event during sleep, just as it does when a person is awake, and adjust ACTH levels accordingly.

In a recent study, he advised one group of volunteers that they would be awakened at nine o’clock in the morning; he told another group that he would rouse them at six. He turned the lights off at midnight and then closely monitored the subjects’ brain activity and ACTH levels as they slept. At 5 AM, ACTH levels increased drastically in the early risers, who had expected to wake up in an hour, but remained low in the late sleepers, who had expected to sleep a few more hours. When Born surprised the late sleepers by waking them up at 6 AM, however, their ACTH levels were still low.

“This is the first evidence that expectation and anticipation, which are associated with conscious wakefulness, intrude into sleep,” says Born. “There may be other processes during sleep, like dreaming, that can be regulated voluntarily.”

Born’s work also demonstrates how some people seem to be able to get up when they need to without using an alarm clock. Waking up is often an act of will, and not an involuntary response. The brain keeps track of time and can adjust ACTH levels on demand. People often report waking just moments before their alarm is supposed to ring. Born’s study not only explains how that is possible, it implies that we could exploit that ability much more than we generally do.

The new findings also emphasize that ACTH gives the body tremendous flexibility in adjusting to sudden changes in sleeping patterns. “This has implications for everybody,” says Born. “If you plan your day, including when you want to wake up, you can be better prepared.”