To test the idea, the researchers compared gene expression in depressed patients who died at different times, and found some similarities. That suggests that the depressed people's body clocks may have been shifted by several hours, the researchers said.
Another analysis, however, found that genes that would be expected to shift together didn't do so in depressed people. That finding suggested that the clocks were disrupted.
In other words, Li said, the problem in depressed brains appears to be both shifting and disruption.
"They seem to be sleeping at the wrong time of the day, and the quality of their sleep is also different from healthy sleep," he said.
The sleep-cycle shift held in patients who had a diagnosis of major depression but who had not taken antidepressants before death, the researchers found, suggesting that it's the disease itself and not the treatment that causes the circadian rhythm problems.
Already, symptoms of insomnia and excessive sleep in depressed people have inspired treatments such as light therapy to try to reset the body clock, Li said. The new research is confirmation that such approaches could work. Researchers might also be able to develop drug treatments to fix the body clock, he said.
"This reinforces the old idea that trying to address sleep cycle is a good practice in diagnosis and in treatment," Li said.
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