A person's sleep habits may help identify a newly proposed subtype of depression linked to the body's internal clock.
In a study of 11,380 Australian adults with a history of depression, about 23% met researchers' criteria for what they called "circadian depression."
The profile included at least three of six features: being an evening person, waking up feeling very tired, sleeping longer during the worst periods of depression, experiencing seasonal changes, having a later midpoint between falling asleep and waking up, and experiencing "social jetlag," a large shift in sleep timing between workdays and days off.
The seasonal changes could involve sleep, mood, activity, appetite, weight or energy.
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The study, published on Oct. 20 in The British Journal of Psychiatry, could eventually help guide more personalized care for patients with depression, according to the researchers.
"Depression is not a uniform illness, and understanding the characteristics of different subtypes means we can give people earlier access to treatments that will be most effective for them," first author Emiliana Tonini, Ph.D., from the University of Sydney's Brain and Mind Centre, said in a statement.
The label, however, is not a formal diagnosis and needs independent replication and further study, the researchers noted.
People in the circadian group reported an earlier onset of depression and more severe symptoms than those who did not meet the criteria. They were also more likely to report weight gain during depressive episodes.
They also reported poorer responses and more side effects with two common classes of antidepressants, selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs).
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Participants reported how well their medications worked, but the study did not explain why some were less effective and did not test treatments aimed at adjusting the body’s internal clock.
Researchers said light exposure, regular sleep schedules and melatonin-related approaches warrant further study.
Dr. Ian Hickie, a study co-author and professor of psychiatry at the University of Sydney, said the proposed profile "cuts across existing historic subtypes" of depression, most commonly atypical and bipolar depression, because they share features such as low energy and oversleeping.
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Atypical depression can also involve weight gain, while bipolar depression refers to depressive episodes in people with bipolar disorder, which also involves periods of unusually elevated or irritable mood and increased energy.
"We define circadian only by sleep-wake and circadian features," Hickie told Fox News Digital.
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For some people with this profile, Hickie said sleep-wake disruption may be more than a symptom of depression. "This type of circadian disturbance is not simply a consequence, but the underlying cause," he wrote.
The study itself, however, could not establish whether body-clock disruption caused depression or resulted from it.
Staying up late or having trouble waking does not necessarily indicate depression. Participants had already reported depression, which in some cases was severe, recurring or long-lasting.
Participants in the circadian group also reported more manic-like experiences. John Puls, a psychotherapist and adjunct professor at Florida Atlantic University, said those symptoms could help explain their reported response to antidepressants.
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"SSRIs are not meant to treat these more significant mental health symptoms that go far beyond depressive symptoms," Puls, who was not involved in the study, told Fox News Digital. "In fact, some SSRIs can actually worsen manic symptoms, which lead to poor sleep."
There were several limitations to the study, the researchers acknowledged.
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Participants reported their own sleep patterns and treatment experiences, and the cross-sectional analysis didn't show whether sleep disruption preceded or followed depressive symptoms.
All participants were of European ancestry, so the findings may not apply to other populations.
Jillian Amodio, a Maryland-based psychotherapist and founder of Moms For Mental Health, cautioned against using the findings as "a benchmark for treating each person in the same way."
"Biological, environmental, social and psychological factors all need to be accounted for," Amodio, who was not involved in the research, told Fox News Digital.
Researchers should examine more populations, including people who work night shifts, and account for differences in time zones and seasons, according to Amodio.
The study was led by researchers at the University of Sydney and QIMR Berghofer Medical Research Institute, with co-authors from institutions in Australia, the U.K. and the U.S.