Night owls who stay up late and sleep in might be able to work longer and harder than early birds, Belgian researchers say.

In Friday's issue of the journal Science, researchers at the University of Liege used brain scans to test the alertness and ability to concentrate in 30 people who were either extremely early or very late risers.

Early birds got up between 5 a.m. and 6 a.m., and the night owls rose at noon.

Participants spent two consecutive nights at a sleep lab, where they periodically did tasks that required sustained attention while sticking with their natural schedules.

Night owls generally outlasted early birds in terms of how long they could stay awake before mental fatigue sent in, the researchers found.

After 10 hours of being awake, early birds showed less activity in brain areas linked to attention and performed the tasks more slowly compared with the night owls. There was no difference in "morning" alertness.

Study author Philippe Peigneux of the Cyclotron Research Centre and his colleagues focused their analysis of the functional magnetic resonance imaging scans on the parts of the brain where our main circadian clock is found.

Our circadian clock helps run day-night schedules by synchronizing the body's rhythms, such as fall of body temperature, with lightness and darkness.

The circadian clock is balanced by a homeostat that tracks how long someone has stayed up to induce sleep when it's been too long.

The study suggested the circadian clocks of morning people are more sensitive to sleep pressure from the homeostatic system in the evening than those of night owls.

Morning larks also fell into deep slow-wave sleep, which relieves homeostatic pressure, more quickly than evening types, the researchers found.