Last week, we examined some of the research on daylight saving time (DST) and the social and economic factors that should be considered before switching to permanent DST. This week we examine the possible health effects of “springing forward” and “falling back.”

There seems to be agreement that most of the health effects of DST result from disruption in the human circadian clock’s seasonal adjustment, messing up chronobiology rhythms. Several sleep scientists believe the impact of DST is poorly understood.

Circadian clocks use daylight to synchronize to the environment, and we adjust to the east-to-west progression of dawn within a given time zone. One study of 55,000 people living in Central Europe showed the quality and duration of sleep were  disrupted when daylight came later in the morning and the effect lasted for several days.[1] 

A commentary in JAMA Neurology lists several studies supporting that springing forward is harder on our bodies, including increased levels of cortisol and sleep problems, especially in adolescents and children.

According to the American Academy of Sleep Medicine (AASM), there is growing evidence that indicates changing time, especially from standard time to DST and back, increases the risks of cardiovascular events, metabolic disease, mood disorders, and other bad health outcomes.[2]

Heart Attacks

A study from the Karolinska Institute in Sweden found “the incidence of acute myocardial infarction was significantly increased for the first 3 weekdays after the transition to daylight saving time in the spring. In contrast, after the transition out of daylight saving time in the autumn, only the first weekday was affected.”[3]

Quality of Life

The Society for Research on Biological Rhythms (SBRR) sponsored an expert review comparing large populations living in DST or standard time, and living on western or eastern edges of time zones. It concluded that it is better to live in permanent standard time than switching or permanent DST.

The European Biological Rhythms Society and the European Sleep Research Society joined with the SBRR to support permanent standard time based on this review of the literature which also looked at acute and chronic effects of DST. Acute effects included heart attack, stroke, hospital admissions for atrial fibrillation, and emergency room visits, as well as missed medical visits and returned visits to the hospital.

The SBRR report stated that although chronic effects have not been studied directly, DST increases the time difference between the social clock (personal and work time) and the body clock. Studies show these time differences can affect life expectancy, sleep, and mental and cognitive problems.

Springing forward means less exposure to morning light and more exposure to evening light. With work and social demands often in early morning hours, this can lead to sleep loss, sleep debt, and circadian misalignment, which can result in various cellular derangements.


A study focused on ischemic stroke found DST appears to be associated with an increase in stroke hospitalizations during the first two days after the switch but not during the entire following week. Susceptibility to effects of DST transitions on occurrence of ischemic stroke may be modulated by gender, age (over 65), and malignant comorbidities (cancer).


There appears to be evidence that circadian disruption could be a human carcinogen. From the east to west of a time zone, social time stays the same while solar time isprogressively delayed. Western time zone residents experience greater circadian disruption as a result, and may be at an increased risk of cancer. One large study found associations between time zone position and age-standardized county-level incidence rates for total cancers combined and 23 specific cancers—including 4 million cancer diagnoses in white residents of 607 counties across 11 states.

The investigation found risk increased from east to west within a time zone for total and for many specific cancers, including chronic lymphocytic leukemia (for both genders) and for cancers of the stomach, liver, prostate, and Non-Hodgkin lymphoma in men, and for cancers of the esophagus, colorectum, lung, breast, and corpus uteri in women.

Mental Health

A study in Denmark based on 185,419 hospital contacts for unipolar depression showed the fall back was associated with an 11 percent increase in unipolar depressive episodes that dissipated over approximately 10 weeks. The spring forward was not associated with a change in unipolar depressive episodes.


Research on time change and obesity has been inconsistent. One large international study found evidence in some settings of longer daylight hours associated with increased activity in boys.

Using data from the Centers for Disease Control and Prevention and the U.S. Census, another group found discontinuity in the timing of natural light affected health outcomes typically associated with circadian rhythms disruptions (e.g., obesity, diabetes, cardiovascular diseases, and breast cancer).

It was disappointing to not see more science used in the development of the Sunshine Protection Act. While the U.S. House of Representatives held a hearing with debate from two sleep experts, the Senate did not do so before their vote.

Given that the House has delayed their vote on the bill, there is a chance policymakers can explore more deeply into the science. The AASM made a list of additional research that would be a good place to start, including studies on the chronic effects of DST on physiology, performance, health, economics, and safety, along with studies addressing confounding seasonal effects, time zone, and differences between effects of permanent standard time and permanent DST.

[1] Kanterman, et al, 2007.

[2] doi: 10.5664/jcsm.8780

[3] N Engl J Med 2008; 359:1966-1968 DOI: 10.1056/NEJMc0807104