Sleep study: Night owls versus early birdsWe all know an oddball night owl who can't fall asleep until day is almost dawning, and then is biologically programmed to sleep till noon. At the opposite end of the spectrum are early birds; wide awake by 4 or 5 a.m., they need alarm clocks like fish need bicycles.
But now we learn that while the early bird may get the worm, they also get sleepier, faster, as their day progresses. The balance between sleep and wakefulness results from a struggle between two counteracting forces: a "sleep pressure" that starts to rise pretty much upon waking, and a wakefulness signal sent by the "body clock" in the brain's suprachiasmatic nucleus, which runs the body's clock that controls our daily (circadian) rhythms of body temperature, immune activity and sleep.
"The circadian signal is low in the morning, and increases during the day, helping to counteract the accumulation of sleep pressure," says Philippe Peigneux, professor of clinical neuropsychology at the Free University of Brussels. "At the end of the day, you have a lot of sleep pressure, and because the circadian signal starts diminishing at that time, eventually sleep can take place."
Although the basic circadian rhythm follows 24-hour patterns, it varies greatly among individuals.
Looking (yawn) at the brainPeigneux guided a new study by Ph.D. student Christina Schmidt at the Cyclotron Research Centre of the University of Liège, comparing brain regulation of sleepiness in early birds and night owls.
To start, Schmidt asked 16 extreme morning types and 15 extreme evening types to follow their natural sleeping schedule for a week, at which point she tracked them for two nights in a sleep lab. She gauged sleep pressure by measuring slow brain waves during the first two hours of sleep, which mark a greater need for sleep in the previous hours.
Morning people had more slow waves, but otherwise Schmidt found the sleep in the two groups to be similar in quality and quantity.
When they were awake, however, an accepted index of sleepiness showed that sleep pressure was building up faster among the early birds. Although reaction speeds -- a measure of general alertness -- were similar in both groups just after waking, the evening types improved their performance as the day wore on. The morning people did not.
In each subject's "morning," at about one and a half hours after their natural wake-up, and in the "evening," after 10 and a half hours awake, the subjects entered a magnetic resonance machine and performed a simple test for alertness.
The MRI machine was set to measure brain activity by tracking oxygen use in various locations. Of special interest to the researchers was the suprachiasmatic nucleus, which is "considered to be the master clock, responsible for the circadian rhythm," says Peigneux.
He knows the dozeThe early birds had less activity in the suprachiasmatic area in the evening session, suggesting that their higher level of sleep pressure was counteracting the circadian signal. "We believe in morning types, because there is more sleep pressure, the alerting signal of the circadian system is hampered" by evening, says Peigneux.
Because sleep pressure built up more slowly among night owls, they were more alert 11 hours after waking.
It may seem that night owls get all the advantages, but Peigneux says this was an artificial situation, where they were directed to obey their internal body clocks. "When evening people are obliged to live in normal society, they usually will have more problems, because they have to adapt to their schedules, and they will have a lot of sleep debt. Many evening people go to sleep late, but they have to wake up because they have children or a job, so on the weekend, they will sleep more to recover from their lack of sleep."
These new results demonstrate the effects of the complex interactions between body regulation and circadian processes upon the brain activity that underlies human behavior, Peigneux explains.
The exact role of sleep has "been a matter of debate for at least 50 years," he adds. "When you fall asleep, you are not conscious of the external world, but sleep helps restore your ability to be alert the next day. Sleep is believed to be important for thermal regulation and memory consolidation, and many other functions. If you have to spend one-third or one-quarter of your life sleep, it probably has more than just one function; that would be quite inefficient."
- David J. Tenenbaum
• Homeostatic Sleep Pressure and Responses to Sustained Attention in the Suprachiasmatic Area, Christina Schmidt et al, Science, 24 April 2009.