Daily variations in homeostatic sleep pressure and circadian alert signal. The relationship between these processes changes according to the "early" or "late" chronotype of an individual. The bottom panel shows suprachiasmatic nucleus (SCA) and locus coeruleus (LC) cerebral areas involved in the alerting signal, whose activity is modulated by differences in experienced homeostatic sleep pressure according to the subject's chronotype. [Image copyright AAAS/Science]
A new brain-imaging study may help explain why some people are most alert early in the day, while others hit their stride in the evening.
Christina Schmidt at the University of Liege in Belgium and her colleagues report that our alertness and ability to concentrate are affected by both how long we've been awake and the time of day, since our circadian rhythms operate according to a day-night cycle. These findings appear in the 24 April 2009 issue of the journal Science.
The researchers used functional magnetic resonance imaging to monitor the brain activity of volunteers who were either early-risers or night owls. The volunteers spent two consecutive nights in a sleep laboratory and periodically performed a task that required sustained attention.
The results suggest that night owls generally outlast early birds, staying awake for longer periods of time before becoming mentally fatigued. After 10 hours of being awake, the early-birds showed reduced activity in brain areas linked to attention, compared to the night owls. They also felt sleepier and tended to perform more slowly on the task.
These brain areas include one that is home to the circadian master clock. Activity in this area decreased the longer the volunteers were awake (i.e., as "sleep pressure" increased). The authors suggest that the behavioral difference between early birds and night owls are at least partly the result of interactions between the brain regions controlling sleep pressure and circadian rhythms.