Researchers found that early risers have higher concentrations of cortisol, a hormone associated with stress, than people who get out of bed later.
"This work is interesting because it may provide a physiological basis for the often-reported difference between early and late risers," said Dr Angela Clow of the University of Westminster in London, who carried out the study.
"Early awakening was associated with greater powers of concentration, being busier and experiencing more hassles through the day, as well as reporting more anger and less energy at the end of the day," Dr Clow said.
"On the other hand, late wakers were more leisurely and less busy. It is possible that cortisol may contribute towards these differences in temperament, as it is known to be able to influence mood and concentration."
The study investigated 42 healthy individuals by taking saliva samples at eight intervals through the day. These were compared against rising times.
Early awakening was significantly linked with higher cortisol levels, but researchers were unable to establish whether higher cortisol caused people to wake up early or vice versa, or whether the link was simply a coincidence.
"Whether early awakening is due to higher cortisol levels or whether higher cortisol levels are the consequence of early awakening may not be the most important question," Dr Clow said. "Regardless of the reason for the different cortisol levels, once in the system this potent hormone will have widespread effects throughout the body."
Dr Clow found that over a 10-week follow-up period, the early risers were more likely to complain of muscle aches, colds, headaches and mood swings.