Early morning people may be genetically determined.
In a recent issue of the journal Cell, University of California (at San Francisco) researchers showed that when a human gene was inserted into an otherwise normal mouse, it caused the mouse to wake up early. A classic sign of familial advanced sleep
phase syndrome (FASPS). (More info is also availabe in this Washington Post article.)
"This study highlights the power of natural human mutations to uncover things (about the circadian clock) that we might not otherwise have learned, or that we might have misunderstood before," said Howard Hughes Investigator Louis Ptacek of UCSF.
FASPS is an inherited condition in which people are "early birds." They rise early and go to bed early. Those with the condition generally show changes in core body temperatures and other characteristics governed by the circadian clock that are shifted up by 3 to 4 hours. It takes just one copy of the abnormal gene to exhibit symptoms.