Canadian researchers say they may have found an objective way to tell if someone is suffering from depression by measuring heart rate abnormalities during sleep.

"I was surprised to see the accuracy of this," said professor Rebecca Robillard, a sleep researcher at the Royal Ottawa Institute of Mental Health Research

One of the problems with diagnosing depression accurately is that it is often subjective. Patients report symptoms such as lethargy and low mood, but doctors have no reliable way to confirm it with an objective test or biomarker. 

The study compared sleep tests conducted on 664 people with diagnosed depression who had sought treatment for sleep disorders. The results were compared with sleep studies done on some 529 people who did not have diagnosed depression.

The data on heart rates during the various stages of sleep was fed to a computer with artificial intelligence programming to spot and learn from differences in the rates and their variation during sleep.

The study found the AI program detected depression accurately in 79.9 per cent of patients. That is almost double the 47 per cent rate of detection by physicians.

"Our preliminary findings in a follow-up study show that there are marked heart rate abnormalities in people with depression, especially during sleep", said Robillard. "The research found faster heart rate and poorer heart rate variability (HRV), two factors linked to poor cardiovascular health."

She said the study also shows these abnormalities seem to be worse during sleep than while people are awake.

“We struggle to make the diagnosis of depression accurately or in timely fashion," said Dr. Roger McIntyre, a professor of psychiatry and pharmacology at the University of Toronto who was not part of the study, but commented on the results.

"What's interesting about this article is that cardiovascular changes in heart rate are identified as a core abnormality in depression."    

The question now is what exactly are the abnormalities detected by the AI program and what is the connection they have to the brain and heart during sleep?

"Is there some sort of miscommunication that drives some of these abnormalities?" said Robillard, who is planning more research into the link.

In the meantime, having biological evidence in the form of a test for depression would help with the stigma that surrounds this mental disorder, Robillard said.

"This highlights depression is not just in the head, it is a whole body medical condition. It's not just psychological.”

She said a sleep test would never replace human judgement but could help doctors make the diagnosis and monitor treatment by seeing the effects on sleep.

Robillard also hopes to test smart watches and other monitoring devices to check heart rate during sleep as more accessible and less expensive ways of assisting with making the diagnosis.