BAKERSFIELD, Calif. -- Do you rise with the crack of dawn or stay up past midnight? Either way, it's likely that your sleep preferences are partly governed by your brain.

Belgian researchers studied extreme night owls and early birds over the course of two days in a sleep lab.

The first morning, volunteers performed a reaction-time test while undergoing a brain scan -- night owls and early birds scored equally well at this point.

But the next day, the participants performed the same test in the evening. This time, the night owls were noticeably faster.

It turned out that early birds had more activity in an area of the brain that houses the master clock. The longer the day went on, the more active the master clock became.

This puts added sleep pressure on the early birds, which may explain why they felt more fatigue.

Experts recommended that early risers save strenuous tasks for the morning and afternoon whenever possible, while night owls may have the luxury of putting their work off until evening if they choose.