It's an uncanny way to start the day: After setting an alarm the night before, you wake up with a start just minutes before it goes off. Coincidence? Probably not, according to research in tomorrow's issue of Nature. Levels of a stress hormone seem to rise about an hour before a sleeper anticipates waking up. The findings may help scientists understand how stress can cause insomnia.
About a quarter of adults can wake up at will, without any help from an alarm clock. Another quarter use alarms as backup but usually wake up on time anyway. (The rest of us either get up with a groan or oversleep.) To see if hormones might control such independence, neuroendocrinologist Jan Born of the University of Lübeck in Germany and his colleagues recruited 15 healthy students between the ages of 20 and 30 to spend 3 nights in their sleep laboratory.
The subjects were hooked up to a catheter that took blood samples every 15 minutes. Lights went out at midnight. On one night the volunteers were told to expect a wake-up call at 6 a.m. On the other two nights, they were told they could sleep until 9 a.m., but on one of those nights they were treated to a surprise wake-up at 6 instead, with the excuse of a technical mix-up.
In those sleepers who expected an early wake-up call, levels of the hormone adrenocorticotropin--known to peak just after waking--began to rise at about 4:30. Those who were surprised with an earlier-than-expected wake-up had no such rise until after they were awakened. During the day, adrenocorticotropin is released in response to stress, and the researchers speculate that the early-morning release might be an adaptation that prepares the body for the stress of waking up.
"People tend to think that we're brain dead when we're asleep," says psychologist William Moorcroft of Luther College in Decorah, Iowa. But the new work is a convincing demonstration that sleepers "can keep cognitive track of time--perhaps even better than when we're awake." But exactly how the brain manages to time this trick is unknown. "They're looking at the bell on the [natural] alarm clock, but where's the clock?" asks neuroscientist Craig Heller of Stanford University.
Born says his next step is to look for electrical patterns of brain activity that might point to the source of the wake-up signal. A better understanding of the process, the scientists say, could help explain the connection between stress and wakeful nights.