A new depression subtype linked to the body clock could give hope to people previously told their illness was untreatable.
University of Sydney researchers said they identified the newly coined "circadian depression" in around a quarter of the more than 10,000 Australians who took part in their recent study.
The features of the depression subtype include sleeping longer when depressed, waking up feeling very tired and being a night owl.
Researcher Ian Hickie said the breakthrough had been a relief for people whose depression had not responded to traditional treatments, including anti-depressants or cognitive behavioural therapy.
"This has actually been empowering and enlightening (for patients) to get out of that category otherwise called treatment resistant depression," the university's Brain and Mind Centre policy co-director said.
Despite common knowledge, he said, the circadian rhythm controlled much more than sleep patterns.
"The co-ordination of those clocks underpins your mood, your metabolism, your concentration, your sleep-wake cycle, and many other key factors," Professor Hickie said.
Treatments for the depression subtype relate to monitoring and the circadian rhythm, also known as the body clock, he said.
This can involve monitoring sleep-wake cycles, for example with a smart watch, and working to make them more regular.
Prof Hickie said there was more research to be done about whether the problem was becoming worse because of lifestyle changes, including people spending their days in the artificial light of offices and staying up later at night.
"If we all go out and live what we call the camping lifestyle, or we live in a really rigid light-dark cycle, you see less of these phenomena," the study's co-author said.
"We really should be out in the light and being active and eating during the day, not at night."
A major trial is also under way regarding melatonin, commonly used for sleep problems, as another treatment option.
Results would be available in a few years, said another co-author Jacob Crouse said.
"Melatonin … is commonly thought of as a sleep hormone, but it's really the chemical signal of darkness that tells the body it's night time," he said.
"It's going to be the definitive answer ... is melatonin an antidepressant medication?"
Postdoctoral research fellow Emiliana Tonini said it was vital people with circadian depression accessed targeted treatment, as they showed more severe symptoms from a younger age.
Dr Tonini, the third co-author, said those with this type of depression had a higher genetic risk of conditions such as ADHD and also physical issues related to metabolism and inflammation.
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