University of Sydney scientists propose an explanation for why antidepressants do not help everyone

The circadian subtype of depression

Depression symptoms and response to treatment vary considerably from person to person. Neuroscientists and psychiatrists at the University of Sydney have suggested that part of these differences is linked to circadian rhythms — the body's internal biological clock, Kun.uz reports. Circadian rhythms are roughly 24-hour fluctuations in physiological processes that adjust to the alternation of day and night; the main external signal that synchronizes them is light. This system plays an important role in sleep and wakefulness, mood, metabolism and concentration, and its disruption can manifest as fatigue and sleep problems resembling jet lag after crossing several time zones.

How the subtype was defined

In a study published in the British Journal of Psychiatry, the researchers analyzed data from 11,380 people aged 18 to 90. Participants were assigned to the circadian subtype if they showed at least three of six features: seasonal changes in mood, an evening chronotype with a tendency to fall asleep late and be active at night, a shift of the midpoint of sleep to a later time, excessively long sleep when the opportunity exists, morning malaise with a feeling of being unrested, and 'social jet lag' — a mismatch between the natural sleep rhythm and the work or study schedule, when sleep lost on weekdays is compensated for on weekends. Based on these criteria, 2,604 people — about 23% of participants — were assigned to the circadian group. The authors emphasize that this is a classification applied within this particular study, and the presence of these features does not in itself constitute a separate diagnosis.

Features of the group

In the circadian group, depression typically began early in life — on average at age 19. The proportion of women was higher, and severe clinical features such as mania, psychosis and suicidal tendencies occurred more often. The group also showed a greater tendency to gain weight during depressive episodes.

Why this matters

Differences in response to antidepressants are a long-standing practical problem in psychiatry, and identifying the circadian subtype offers a possible explanation for part of this variability: if a quarter of patients have a course of depression tied to sleep-wake rhythms, this could influence the choice of therapy. For readers, the practical implications remain limited for now: the authors themselves note that the identified group is a research classification, not a new diagnosis, and the publication contains no data on how treatment should be changed for such patients. It is also unknown whether the approach will be validated in clinical practice.