[cnn-photo-caption image=http://i2.cdn.turner.com/cnn/2009/images/07/09/woman.sick.bed.art.jpg caption="Some people who appear to have insomnia are biologically night owls, one expert says."]
(CNN) - If you have a hard time crawling out of bed in the morning, it could be that your body is biologically programmed to start the day later.
Experts say a spectrum of natural sleeping and waking rhythms exists, ranging from extreme morning people to extreme "night owls."
A new study examines how morning people compare with night owls on a strength test and looks at what other physiological processes may contribute to their performance.
Researchers tested participants' leg muscle strength at various points in the day. They looked at nine "early birds" and nine "night owls," who were classified as such based on a questionnaire.
Surprisingly, morning people's strength tends to remain constant throughout the day, but night owls have peak performance in the evening, said researchers from the University of Alberta in Canada.
"We thought that morning people would be better at this in the morning, but they never changed," said study co-author Olle Lagerquist, a Ph.D. candidate in neurophysiology at the University of Alberta.
That may be because evening people show increased motor cortex and spinal cord excitability in the evening, about 9 p.m., meaning they had maximal central nervous system drive at that time, Lagerquist said.