Australian researchers have identified a proposed subtype of depression linked to disruptions in the body’s internal clock, which they call circadian depression. The findings suggest that this form of depression may respond less effectively to conventional antidepressants, potentially making treatments focused on sleep-wake cycles and circadian rhythms an important area for further research.
The study, published in the British Journal of Psychiatry, analyzed clinical and genetic data from 11,380 Australians, most of whom experienced severe and recurrent depression. Researchers identified features consistent with circadian depression in approximately 23% of participants.
Although the relationship between disrupted daily rhythms and mood disorders has been recognized for years, the researchers argue that their findings strengthen the evidence for a distinct biological subtype that could eventually help clinicians tailor depression treatment more effectively.
What Is Circadian Depression?
Circadian depression refers to a proposed subtype of depression associated with disruptions to the body’s internal timing system, which regulates sleep, wakefulness and other daily biological processes.
Dr Emiliana Tonini, the study’s first author from the University of Sydney’s Brain and Mind Centre, explained that participants were classified as having circadian depression if they exhibited at least three of six characteristics:
- Experiencing a seasonal pattern in their depressive symptoms.
- Going to sleep and waking later than usual.
- Sleeping longer during depressive episodes.
- Identifying as a night owl.
- Waking up feeling particularly tired.
- Experiencing social jetlag, meaning their sleep schedules differ substantially between workdays and days when they can organize their time freely.
According to Tonini, participants classified within this subtype experienced more severe mental health symptoms than those with non-circadian depression. They also tended to develop depression at a younger age, experience more episodes and follow more complex illness trajectories.
The group additionally showed higher levels of suicidality, hypomanic symptoms and psychotic-like symptoms, highlighting the clinical complexity associated with the proposed subtype.
Genetic Findings Point to Distinct Biology
The researchers found that participants with circadian depression had higher genetic risk scores associated with attention deficit hyperactivity disorder (ADHD) and mood disorders. They also identified differences in several metabolic and inflammatory markers.
These findings suggest that circadian depression may involve biological mechanisms beyond sleep disruption alone. The researchers believe this distinction could help improve how depression is classified and treated in the future.
However, the study has important limitations. Participants were restricted to individuals of genetically inferred European ancestry, which may limit how broadly the results can be applied. The researchers emphasized that additional studies involving more diverse populations are needed to replicate the findings.
Why Conventional Antidepressants May Be Less Effective
One of the study’s key findings concerned the response to standard antidepressant treatments. Selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs), commonly used as first-line medications for depression, were associated with lower effectiveness and more side effects among participants with the circadian subtype.
Professor Ian Hickie, a psychiatrist and co-director of the Brain and Mind Centre, said that the subtype’s poorer response to standard therapies was particularly important because these treatments are commonly recommended in clinical guidelines.
The researchers suggest that identifying circadian depression in clinical settings could eventually support more personalized treatment approaches, particularly for young people seeking mental health support for the first time.
Could Light Therapy and Sleep Routines Help?
Researchers suggest that treatments targeting circadian rhythms could be relevant for people with this subtype of depression. Potential approaches include melatonin and light therapy, alongside strategies that promote consistent daily routines.
Hickie highlighted several areas of interest, including:
- Morning light exposure: Incorporating daylight exposure early in the day.
- Regular daytime activity: Maintaining consistent activity patterns during daylight hours.
- Consistent sleep schedules: Establishing regular times for going to bed and waking up.
- Regular meal schedules: Maintaining predictable eating times throughout the day.
These approaches focus on stabilizing the sleep-wake cycle and other daily rhythms. However, further research is required to establish which treatments are most effective for this proposed subtype and how they should be incorporated into clinical care.
Experts Call for Further Research
Professor Jayashri Kulkarni, director of the Monash Alfred Psychiatry Research Centre, described the study as well conducted. She highlighted the use of polygenic risk scores, which helped researchers examine associations between depression characteristics and genetic profiles.
Kulkarni said these findings could be important for future treatment development. However, she also noted that the study did not account for hormonal changes associated with menopause, which can contribute to sleep disturbances in women. She suggested this could be a valuable factor to investigate in future research.
Professor Andrew Coogan, a behavioral neuroscientist at Maynooth University in Ireland, noted that the relationship between major depression and changes in the body clock has been recognized for some time. People with depression may experience shifts toward later bedtimes and wake-up times.
Coogan questioned how the researchers developed the criteria used to identify the proposed subtype. He described the findings as interesting preliminary results that require further examination rather than definitive evidence establishing a new clinical category.
What the Findings Could Mean for Depression Treatment
The study adds to research exploring how biological rhythms, genetic factors and metabolic processes may contribute to different patterns of depression. By identifying a group of patients with shared characteristics and potentially different treatment responses, researchers hope to improve the precision of future mental health care.
Further studies will be needed to validate the proposed subtype across diverse populations, clarify its underlying biology and determine whether circadian-focused treatments can improve outcomes.
For now, the findings highlight the potential importance of sleep timing, daylight exposure and daily routines in understanding depression, while underscoring the need for additional evidence before the proposed subtype can be fully incorporated into clinical practice.
