Researchers Think Sleep Could Be Behind A New 'Circadian' Depression

About 20% of adults in the UK experience more “common” mental health conditions, like depression and anxiety. And we know that depression can change how you sleep – disturbed sleep, insomnia, “hypersomnia” (sleeping a lot), and fatigue are the “most consistent symptoms associated with major depressive disorder”.

But since 2021, researchers have thought that depression heavily linked to our body clock could form its own form of the condition – “Circadian depression”. 

A new paper published in the British Journal of Psychiatry in October of 2026 has backed this theory up further.

What is “circadian depression” and what are its proposed symptoms? 

In its most basic terms, the proposed depression subtype is a variant of the mental illness in which a person’s natural sleep/wake cycle is significantly disrupted. 

The original 2021 paper suggested “circadian depression” could have the following symptoms: 

A disrupted 24-hour sleep-wake cycle,Reduced motor activity (movement),Low subjective energy, andWeight gain.

Researchers for that same study said “circadian depression” could follow a distinct course, too. It seems to follow certain patterns like: 

Starting at an earlier age than other kinds of depression,Phenomena “suggestive of bipolarity”, possibly linked to how people’s real levels of movement may affect their mood or energy and vice versa,Not responding very well to conventional antidepressant medications, andExperiencing cardiometabolic and inflammatory disturbances at the same time as depression.

But in the 2026 research, scientists were only looking for three or more of the following “binary circadian-associated features” among participants with depression: 

Social jetlag, or having a sleep pattern out of sync with their daily lives, perhaps due to e.g. very long lie-ins on the weekend,Seasonality (recurring depression that followed a predictable seasonal pattern),Delayed sleep midpoint, meaning the “middle” of their sleep was later than average; this suggests a later bedtime, Evening chronotype, or being a night owl,Morning tiredness, andHypersomnia.

Why do researchers think “circadian depression” could be its own diagnosis?

Proponents say it seems to have its own symptoms and follow a different course to other forms of depression, as we’ve outlined above.

Suspected “circadian depression” also might respond better to body-clock-specific treatments, like melatonin, melatonin-targeted antidepressants and light therapy, even when they respond poorly to serotonin-based antidepressants.

In the 2026 paper, researchers analysed the data of over 10,000 people with depression involved in the Australian Genetics of Depression Study. 

Almost 25% of these seemed to display at least three traits that the researchers believe are linked to “circadian depression”, like being a night owl or waking up feeling really tired.

People in this group also reported an earlier onset of their depression and seemed to face symptoms like mania and a higher risk of suicidal thoughts, while their genes seemed more likely to prime them for inflammatory and cardiovascular health problems as well as neurodevelopmental disorders like ADHD.

The study’s first author, the University of Sydney’s Dr Emiliana Tonini, said: “Our research has identified circadian depression as a new subtype, affecting around a quarter of people in our study into those with severe, recurrent or persistent depression.

“Depression is not a uniform illness and understanding the characteristics of different subtypes mean that we can give people earlier access to treatments that will be most effective for them.” 

Meanwhile, the study’s co-senior author, Dr Jacob Crouse (also from the University of Sydney’s Brain and Mind Centre), added: “Because circadian depression is linked to people’s internal body clock, treatments targeting the circadian rhythm, such as light therapy and melatonin, may be much more relevant, and clinicians should reach for these much earlier in people with this presentation.

“For those who find that antidepressants are not working for them, it is important to speak to your clinician for advice about your individual circumstances”, he ended.

Help and support:

Mind, open Monday to Friday, 9am-6pm on 0300 123 3393.Samaritans offers a listening service which is open 24 hours a day, on 116 123 (UK and ROI - this number is FREE to call and will not appear on your phone bill).CALM (the Campaign Against Living Miserably) offer a helpline open 5pm-midnight, 365 days a year, on 0800 58 58 58, and a webchat service.The Mix is a free support service for people under 25. Call 0808 808 4994 or email help@themix.org.ukRethink Mental Illness offers practical help through its advice line which can be reached on 0808 801 0525 (Monday to Friday 10am-4pm). More info can be found on rethink.org.