Professor Ian Hickie, co-director of the Brain and Mind Centre at the University of Sydney and a co-author of the study, said the findings fundamentally reframe the relationship between the body clock and depression. 'Our fundamental starting position is, for a significant part of the population... it's the other way around,' Hickie said. 'It's the disturbed body clock that is actually driving the depression. If it's not attended to, you won't fix the episode of depression, and you won't prevent its longer-term recurrence.'
Individuals with the circadian subtype report at least three of six criteria: seasonal changes in their illness, long sleep during depressive episodes, morning grogginess, being a night owl, waking late, and experiencing social jet lag. The researchers noted that women and individuals whose symptoms began in adolescence or early adulthood were most commonly affected, with 80 per cent of the identified subgroup being female. Professor Sean Cain of Flinders University, who was not involved in the study, observed that people with severe depression are three times more likely to use darkness metaphors, which he linked to a faulty brightness detection system in the circadian system.
The study's authors caution that SSRIs increase the circadian system's sensitivity to light. For night owls and others exposed to bright light in the evenings, this heightened sensitivity can worsen circadian disruption and potentially exacerbate depressive symptoms.
Professor Michael Berk, chair of psychiatry at Deakin University, called the research 'a very important start' in defining a new subtype but said further study is needed before it can definitively guide clinical practice. Lead author Dr Emiliana Tonini suggested that future treatment may include melatonin or melatonin-targeted antidepressants, along with behavioral approaches such as waking within the same one-hour window each day and carefully managing light exposure, maximising daylight and dimming lights at night.