But seeing whether that's true in the human brain is a lot tougher, said researcher Huda Akil.
If you want to look for daily rhythms in hormone activity, Akil
said, you can take multiple blood samples from the same people over the course of 24 hours.
You cannot, however, investigate the brain that way.
To get around the problem, Akil's team studied autopsied brain tissue from 89 people who had died at different times of day.
That way, they could look at each person's gene activity at the time of death and search for differences from one individual to the next.
"Hundreds of genes emerged as having a rhythm based on the time of day," said Akil, a professor of neuroscience and psychiatry at the University of Michigan in Ann Arbor.
That rhythm was clear in brain tissue from the 55 people with no history of psychiatric disorders.
team was able to look at an individual's gene activity and correctly guess that person's time of death within an hour.
They could not, however, do that for the 34 individuals who were suffering from major depression at the time of death.
Their gene activity patterns were too varied.
"This is very clear evidence that the 'clock' in the brain is disrupted in depression," Akil
Either way, Akil
thinks the out-of-sync genes would feed people's symptoms.
Think about how bad you feel, she
said, when the body's normal rhythms are thrown off due to jet lag.
In the future, Akil
said, the findings might help lead to new "biomarkers" for diagnosing depression or tracking how well the disorder is responding to treatment.
The findings might also help identify new "molecular targets" for depression treatment, Akil
Right now, antidepressant drugs target certain chemicals in the brain believed to contribute to depression -- most famously, the mood-regulating chemical serotonin.
said the new findings show that there are multiple things going wrong in the brain when a person has major depression.
"It's not only a serotonin imbalance," she