Wake me up before you go go 

SARA ABDULLA

Why can some people get themselves up at a certain time without an alarm clock? Or why, on the one day of the month that you could get the lie-in you so desperately need, do you find yourself wide awake at 6.45 as if it were a working day? And why is it that being prematurely woken can leave you feeling jumpy, even if you've slept enough? 

It's because, as Jan Born and colleagues of University of Lübeck, Germany report in Nature, the expectation that sleep will come to an end at a certain time induces an increase of about 30% in the concentration in the blood of two 'stress-response' hormones, adrenocorticotrophin and cortisol, one hour before your wake-up call. 

Levels of adrenocorticotrophin and cortisol are known to increase throughout sleep, reaching a daily maximum at the time of spontaneous waking. They are released by the pituitary gland as part of the 'fight or flight' response to stress that ups heart rate, increases blood flow to the muscles, stimulates the brain and raises the metabolic rate. So what these new findings suggest is that even when asleep, the body apparently has a finely tuned way of preparing itself for the stress of getting up. 

Born's group studied fifteen healthy volunteers with regular sleep patterns over three consecutive nights. Before midnight lights-out on two of the nights, subjects were told that they would be woken at 9am – what the researchers called the 'long sleep' condition. On the other night they were informed that the wake up call would come at 6am ('short sleep'). On one of the long sleep nights, however, subjects were woken, not at 9am as they were expecting, but at 6am, "under the pretence of a technical fault". During all this the researchers took blood samples every 15 minutes to investigate the subjects' hormone levels. 

In all the situations, hormone concentrations remained similar until around 4.30am. Thereafter, however, differences began to emerge. "When anticipating being woken up at 6am, subjects showed a distinct increase in adrenocorticotrophin levels within the last hour before waking compared with under surprise conditions," says Born. Which indicates that anticipation, a trait generally thought to be a unique characteristic of conscious action, pervades sleep. 

But in addition to providing 'control' data for the short sleep condition, the recordings from subjects aroused early from long sleep, revealed that, even if caught unawares, the body can adjust quickly to a new situation. For in these trials the researchers found a temporary increase in the concentrations of adrenocorticotropin and cortisol, which peaked about 30 minutes after subjects had been wrenched from sleep. Small wonder most of us hate mornings. 

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