Seasonal Affective Disorder (SAD) has long been described as a predictable, calendar-linked form of depression: low mood, fatigue, social withdrawal, and carbohydrate cravings that emerge each fall or winter and ease in spring. Clinically, it’s treated as a seasonal specifier of major depressive disorder, with light therapy, antidepressants, and psychotherapy offered to those who report recurring, seasonally-timed episodes. Early estimates and clinical lore suggested a nontrivial fraction of people experience mild seasonal mood change (sometimes cited in single-digit to low-teen percentages) with the explanation focused on reduced daylight disrupting circadian timing and neurotransmitter balance.

A large 2016 population study, however, challenged that prevailing view. Researchers analyzed more than 34,000 U.S. adults from the Behavioral Risk Factor Surveillance System using the PHQ-8 symptom inventory and linked responses to season, latitude, and sunlight exposure. They found no systematic rise in depressive symptoms during winter months, no latitudinal gradient, and no relationship with measured sunlight. Even among respondents meeting clinical cutoffs for depression, seasonal differences were absent. The authors argued that being depressed in winter does not prove causation by winter and that recall bias and nonstandard diagnostic practices in earlier research may have amplified perceived seasonality.

How to reconcile the views? The most likely resolution is nuanced. True, medically-meaningful seasonal depression probably exists for a minority of individuals, but it is not as pervasive across the general population as cultural narratives imply. The methods used in these studies matters. With prospective symptom tracking, objective sleep and light measures, and repeated assessments across years needed to confirm seasonality in individuals.

For clinicians and patients the takeaway is practical. Treat current depressive symptoms regardless of presumed seasonality, but verify repeated, calendar-linked patterns before prescribing long-term seasonal treatments. For researchers, the priority is longitudinal studies and targeted trials to identify who genuinely benefits from timed light therapy or other season-specific interventions.