Human beings have their own natural alarm clocks which can be set subconsciously just by anticipating the time we want to wake up.

Research conducted by scientists at the University of Lubeck in Germany has found that people release hormones before waking which help them to anticipate the "stress" of getting up at different hours.

Their work has also revealed the importance of the power of thought when it comes to controlling our sleep patterns.

According to Professor Jan Born, one of the researchers, a surge of hormones is our body's way of saying "wake up!" - but only when we are expecting to wake up.

The researchers measured the levels of two stress hormones - adrenocorticotropin (ACTH) and cortisol - in the blood of two sets of people as they slept.

Power of thought

Previous research has shown that people's bodies have their own rhythms which can help them wake at regular times each day, but - says Professor Born - this is the first real evidence of an internal "mechanism which quickly adapts the sleep-wake rhythm to environmental challenges."

Scientists believe this greater understanding of sleep could allow scientists to develop ways of improving the lot of shift workers whose sleep patterns are constantly being disrupted.

Fifteen healthy volunteers with an average age of 25 and regular sleep patterns took part in the experiment, the results of which have been published in this week's Nature magazine.

The volunteers were studied on three nights, on two of which 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

When the volunteers knew they had to get up early, their levels of the ACTH hormone 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 - which is released in response to stress - after waking, suggesting they had a more difficult time waking up.

The researchers found that increased levels of the two hormones were also present before waking in people who had been told that they would only sleep for a short period.

Writing in Nature magazine, the researchers concluded that the increase in hormones indicates that anticipation, which is widely thought to be a characteristic unique to conscious action, also pervades sleep and may facilitate spontaneous waking.