Early to bed, early to rise makes a man sleepy and inattentive at twilight.
A new brain imaging study suggests morning people’s circadian clocks can’t resist the biological pressure to sleep, while night owls don’t buckle as easily. The research, appearing in the April 24 Science, could change the way scientists view the relationship between sleep and the circadian clock.
Two systems control sleeping and waking — the circadian clock and the sleep homeostat. The circadian clock helps synch the body’s rhythms, such as the rise and fall of blood pressure and body temperature, with light and dark cycles. The homeostat is a biological accountant that keeps track of how long a person has been awake or asleep and how much sleep the person has had recently — inducing sleep when it’s been too long.
As the day goes on, the waking signal from the clock gets stronger, says Derk-Jan Dijk, a professor of sleep and physiology at the University of Surrey in Guildford, England. “It’s not just an alarm clock that gives one signal in the morning,” he says. “It’s really a process.” If the circadian clock operated alone, people would be most awake in the evening, but the homeostat counterbalances the clock’s wake signal.
Until now, scientists have either viewed the two systems as separate, or assumed that the clock was the most important factor in determining bedtimes. But the study introduces a new, more intimate link between the two systems. The homeostat plays a more important role than people thought in determining sleeping and waking times, says Allan Pack of the University of Pennsylvania School of Medicine in Philadelphia.
Christina Schmidt of the University of Liège and Philippe Peigneux of the Université Libre de Bruxelles, both in Belgium, and colleagues in France and Switzerland studied people with extreme bedtimes, or chronotypes, both early and late. The larks in the study typically woke up between 4 a.m. and 5:30 a.m. and went to bed by 9 p.m. The night owls, or evening chronotypes, left to their own devices would go to bed at 3 a.m. or 4 a.m. and rise at noon. Many night owls don’t follow their natural schedule, though, Peigneux says. “Their problem is that they are at complete contradiction to the work schedule of society,” he says.
Chronotypes follow a bell-shaped curve, with most people — 66 percent — falling in the neutral zone. The volunteers in this study represent the 5 percent to 10 percent of people at either end of the curve.
Volunteers slept according to their own natural cycles for a week and then spent two nights in a lab, where lights were kept dim to avoid interference with circadian rhythms. Researchers did two fMRI scans of each person, one given two hours after waking and another 10.5 hours after waking. During scanning, the volunteers completed a mental task that gages alertness.
Scheduling the scans according to how many hours the volunteers had been awake, rather than choosing a particular time of day, allowed the researchers to examine everyone at the same point in their circadian rhythms. The researchers also monitored how much time the volunteers spent in slow-wave and REM sleep.
During the “morning” sessions, researchers saw no difference between the two groups in alertness, and neither group reported feeling sleepy. But in the “evening” sessions, the early birds began to feel tired and performed slower than the night owls. When the researchers examined activity in the suprachiasmatic area, the part of the brain that houses the master circadian clock, they found it was more active during the evening scan in night owls than in larks, suggesting that the clock was sending a stronger wake signal. The locus coeruleus, a part of the brain that is connected to the clock and helps keep people awake, was also more active in evening types.
Morning people’s circadian clocks are more sensitive to sleep pressure from the homeostatic system in the evening than the night owls, the researchers found.
And morning types dropped into deep slow-wave sleep sooner after falling asleep than evening types did. Slow-wave sleep is the stage of sleep that relieves homeostatic pressure. Morning types shook off the sleep pressure faster during slow-wave sleep than night owls did.
The more intense slow-wave sleep that larks experienced at night correlated with less activity in the circadian clock during the next evening scanning session. The result indicates that the sleep homeostat hinders the clock’s ability to keep people awake. The effect is more extreme in morning people, jeopardizing their ability to stay alert in the evening, the researchers conclude.
Peigneux says both chronotypes would do best to find jobs that fit their sleep schedules.
“I would prefer to have a late chronotype doing the night shift in a nuclear plant, for instance,” he says.