Scientists at the University of Lubeck say people release hormones before waking which help them to anticipate the "stress" of getting up at different hours.

Previous research has shown that people's bodies have their own rhythms which can help them wake at regular times each day.

Normally levels of adrenocorticotropin and cortisol, hormones released by the adrenal and pituitary glands, increase during the late stages of sleep.

The researchers found that increased levels of adrenocorticotropin, which stimulates the secretion of cortisol, were also present before waking in people who had been told that they would only sleep for a short period.

They tested 15 healthy volunteers, whose average age was 25 and who had regular sleeping patterns.

They were studied on three nights. On two nights they were told they would sleep until 9am. The researchers let them sleep until 9am on one night, but woke them at 6am on the other.

On the third night, they told the volunteers they would sleep until 6am.

Stress of waking

They tested their sleep patterns throughout the night using electronic equipment and took blood samples every 15 minutes to determine hormone levels.

When the volunteers knew they had to get up early, their levels of adrenocorticotropin increased substantially in the last hour before waking, compared with when they did not know they were being woken at 6am.

After waking on all three nights,the volunteers' hormone levels increased temporarily, peaking 30 minutes later.

The researchers believe this is a response to the "stress" of waking.

Those who were woken at 6am without knowing they would be had higher concentrations of cortisol - a hormone released in response to stress - after waking.

This suggested they had a more difficult time waking up.

Writing in Nature magazine, the researchers concluded: "The increase in adrenocorticotropin release before the expected time of waking indicates that anticipation, which is generally considered to be a unique characteristic of the regulation of conscious action, pervades sleep.