LIEGE, BELGIUM - Early birds may get a head start in the morning, but it's the night owls that have the staying power, a new brain-scan study reveals.
Scientists led by Christina Schmidt of the University of Liège in Belgium pitted morning larks against night owls in a task designed to measure their reaction and attention times. The team also took hourly saliva samples to measure the sleepers' levels of melatonin, a hormone which regulates sleep cycles.
During the experiment, the volunteers got up and went to bed at their usual times, with the early birds tending to turn in four hours earlier than the night owls.
The two types performed pretty similarly an hour and a half after getting up. But nine hours later, the night owls were both quicker and more alert at the task.
Despite having been awake for the same length of time, the early birds felt sleepier, and scans revealed that the parts of their brains linked to attention were less active.
The difference, the researchers say, was a result of the shift in the balance between the two mechanisms that control alertness: the light-triggered circadian signal and the buildup of the pressure to sleep through the day - the homeostatic process. As the day wears on and the time since sleep becomes greater, the pressure to sleep mounts; at the same time, the continued daylight triggers the circadian signal that promotes wakefulness.
As the day wore on, the night owls showed increased activity in two parts of the brain that are involved in regulating the circadian signal late in the day — the suprachiasmatic nucleus area and the locus coeruleus. The circadian signal was overriding the pressure to sleep. In the early birds, on the other hand, sleep pressure appeared to prevent the expression of the circadian signal.
While researchers had thought that the two systems operated independently, the study found that there was a strong interaction.
The researchers now hope to investigate how these differences affect other cognitive tasks, such as learning and memory.