Summary: A new register-based study from Denmark finds a measurable increase in depression diagnoses following the annual transition from daylight saving time to standard time.
Source: Aarhus University.
Transition to standard time linked to a rise in depression diagnoses
A nationwide register study in Denmark reports a clear short-term rise in hospital-diagnosed depression immediately after the shift from daylight saving time (summer time) back to standard time. The analysis shows that diagnoses of unipolar depressive episodes are higher in the month following the autumn time change than would be expected from existing trends, with an increase on the order of eight to eleven percent in the weeks after the transition.
The researchers examined 185,419 hospital contacts for unipolar depression recorded in the Danish Psychiatric Central Research Register between 1995 and 2012. The study team — a collaboration between psychiatric and political science departments at Aarhus, Copenhagen and Stanford universities — used time-series intervention analysis to compare the observed incidence of depressive episodes after clock changes with the predicted incidence based on the pattern before the transition.
Associate Professor Søren D. Østergaard of Aarhus University Hospital (Department of Clinical Medicine, Aarhus University), one of the lead authors, emphasizes that the increase is unlikely to be coincidental. The rise in diagnoses after the transition to standard time remains statistically robust after accounting for seasonal factors such as day length and weather, which the analyses explicitly control for.
“We are relatively certain that it is the transition from daylight saving time to standard time that causes the increase in the number of depression diagnoses and not, for example, the change in the length of the day or bad weather. In fact, we take these phenomena into account in our analyses,” said Søren D. Østergaard.
Although the data come from psychiatric hospitals and therefore reflect more severe clinical presentations, the authors note there is no strong reason to believe only severe depression is affected. The team expects the full severity spectrum of depressive symptoms to be influenced by the sudden time change. Given the high population prevalence of depression, even an eight-to-eleven percent relative increase corresponds to a substantial absolute number of additional cases in the population.
The study does not pinpoint a single causal mechanism but offers plausible explanations. In Denmark, reverting to standard time effectively shifts one hour of evening daylight into the morning. As a result, evenings become darker earlier while morning daylight hours coincide with many people’s pre-work routines, when they are less likely to benefit from natural light exposure. The abrupt earlier sunset also serves as a psychological signal that darker, colder months are beginning.
“We probably benefit less from the daylight in the morning between seven and eight, because many of us are either in the shower, eating breakfast or sitting in a car or bus on the way to work or school. When we get home and have spare time in the afternoon, it is already dark,” Østergaard explained, adding that the marked advancement of sunset may itself contribute to increased distress.
Clinical and public health implications
The researchers recommend heightened awareness among clinicians, caregivers and people prone to mood disorders in the weeks after the switch to standard time. Increased vigilance for depressive symptoms, timely support and early intervention during this vulnerable period could help reduce the clinical and societal burden of additional cases triggered or exacerbated by the time change.
“Our results should give rise to increased awareness of depression in the weeks following the transition to standard time. This is especially true for people with a tendency towards depression — as well as their relatives. Furthermore the healthcare professionals who diagnose and treat depression should also take our results into consideration,” said Østergaard.
Source: Søren Dinesen Østergaard – Aarhus University
Image Source: NeuroscienceNews.com image (illustrative).
Original Research: Abstract for “Daylight savings time transitions and the incidence rate of unipolar depressive episodes” by Hansen, Bertel T.; Sønderskov, Kim M.; Hageman, Ida; Dinesen, Peter T.; and Østergaard, Søren D., published online in Epidemiology (October 20, 2016), DOI: 10.1097/EDE.0000000000000580.
Aarhus University. “Transition From Daylight Saving Time To Standard Time Leads To Depressions.” NeuroscienceNews. October 29, 2016.
Abstract (study summary)
Background: Daylight saving time transitions affect about 1.6 billion people worldwide. Prior research has linked these transitions to various adverse health outcomes. Because shifts in clock time disrupt circadian rhythms — which are implicated in the development of depressive disorders — this study investigated whether daylight saving time transitions alter the incidence rate of unipolar depressive episodes.
Methods: Using time-series intervention analysis on nationwide Danish Psychiatric Central Research Register data from 1995 to 2012, the study compared observed incidence rates of hospital contacts for unipolar depressive episodes after transitions to and from summer time with the predicted rates based on pre-transition trends.
Results: The analysis, based on 185,419 hospital contacts for unipolar depression, found that the transition from summer time to standard time was associated with an approximately 11% increase (95% CI: 7%–15%) in the incidence rate of unipolar depressive episodes. This elevation gradually dissipated over about ten weeks. The transition from standard time to summer time showed no comparable effect.
Conclusion: Transitioning from daylight saving time to standard time is associated with a transient increase in hospital-diagnosed unipolar depressive episodes. The sudden earlier onset of evening darkness and the psychological signal of a long period of short, dark days may help explain this finding. Clinicians and public health professionals should be aware of this increased risk window and consider it when monitoring and supporting vulnerable individuals.