The discovery shows that waking up from a night's sleep can be consciously controlled so individuals can force themselves out of bed if they really have to.
A study of a group of healthy volunteers has shown that the body's alarm clock begins to alert sleepers to the anticipated waking-up time about an hour beforehand.
Rising levels of adrenocorticotropin, a hormone released during the day to deal with stress, start to prepare sleepers for the biological wake- up call, according to Jan Born, professor of neuroendocrinology at the University of Lubeck in Germany.
When the volunteers were told to wake up at 6am, their hormone levels began to rise about an hour beforehand, but when told they would have to wake at 9am, hormone levelsbegan rising at 8am.
"The regulation of adrenocorticotropin release during nocturnal sleep is therefore not confined to daily rhythms; it also reflects a preparatory process in anticipation of the end of sleep," Professor Born and his colleagues report in the journal Nature.
Adrenocorticotropin is known to prepare the body for a stressful event during the day and now seems to be involved in getting the body ready for the "stress" of waking up, Professor Born said.
"This system is suppressed in the early hours of sleep butbecomes activated in the later hours of sleep, just before someone wakes up. It is a completely new view of sleep," he said.
What makes the finding important is the discovery of an element of conscious control over when the hormone is released - the results show people can clearly distinguish between an anticipated sleep time of either six or nine hours.
"The increase of adrenocorticotropin release before the expected time of waking indicates that anticipation, generally considered to be a unique characteristic of the regulation of conscious action, pervades sleep," the researchers report.
Professor Born said there must be a biological mechanism controlling the clock, to inform the body about how much time has been spent asleep, but "it must be a very slow-acting clock and we have no idea what it could be," he said.
The next stage of the research is to determine how brain activity can influence the release of the hormone which could enable the researchers to devise a way of helping people who find it difficult to wake up in the morning.