Do Daylight Savings Changes have Negative Health Effects?

We know that daylight saving time and its reversal in the fall changes our clocks. But does it change our body clocks and does it have negative effects upon our health?

This question is asked by many of us just about every time we have to put up with changing our clocks — putting them an hour forward in the spring and back an hour in the fall. Daylight saving time was instituted in the U.S. during World War I to save energy during wartime production. Today the dates have changed slightly but most of the Northern hemisphere adjusts their clocks forward in the spring and back in the fall without any clear understanding of its effects upon our health.

And most assume that daylight saving time saves energy but this is debatable — as the loss of evening light during the winter cancels any gains made during the summer.

But there is another more critical effect that these changes have upon us: They change our body biorhythms, including our production of cortisol and our production of melatonin.

The effect upon our cortisol levels was recently proven in a study by researchers from Australia’s Queen Elizabeth II Medical Center. The researchers analyzed  27 569 panels of blood cortisol taken during different seasons over a period of thirteen years. The researchers monitored the relationship between the cortisol levels and the season, and whether it was taken during daylight saving time or not.

Their exhaustive study found that cortisol levels indeed do change with the seasons. Cortisol increases 3% in the summer equinox and 4.5% in the fall, to a height of almost 9% increase during the winter solstice. The research indicates that as the sunrise is delayed, cortisol levels rise. The researchers said:

“For each hour later that the sun rose there was an almost 5% increase in median cortisol.”

The research also found that daylight savings time in itself also changed the morning peak of cortisol by an hour — meaning that the body clock did not effectively adjust to the hour daylight savings time.

Does daylight savings time affect sleep quality?

While a slight change in the body’s cortisol levels and the delay of peak levels by an hour due to daylight saving time doesn’t seem that critical, it seriously effects our sleep rhythms, and produces an increase in sleep disturbance.

And as numerous studies have also shown, sleep disturbances are linked to dementia, heart disease, automobile and workplace accidents and many other aspects of our lives.

This effect upon our sleep patterns was shown in a study by researchers from Germany’s Ludwig-Maximilian-University. The researchers conducted a survey of 55,000 people that allowed an understanding of sleep quality with relation to daylight savings time and the reverse in the fall.

The researchers found that while most people were able to adjust to the fall change, reversing daylight savings, they have a tough time adjusting to daylight savings time, and the result is poor quality sleep and sleep disturbances.

The researchers also conducted a second study that followed 50 subjects for eight weeks around daylight savings time and the reverse of the clock during the fall. They researchers studied their sleep quality and timed their sleep length during the two clock transition periods.

In this second study again the researchers found that the subjects did not adjust well to the daylight saving time change in the spring, though they adjusted better during the fall change.

This of course is to be expected, because during the daylight savings time change, the clock is advanced forward, and this forces a loss of an hour, a later sunrise and a later sunset — both of which rearranges our body clock to the negative. In the fall, this loss of an hour is gained back, allowing an earlier sunrise, which effects our cortisol levels positively.

The researchers summarized:

“Our data indicate that the human circadian system does not adjust to DST and that its seasonal adaptation to the changing photoperiods is disrupted by the introduction of summer time. This disruption may extend to other aspects of seasonal biology in humans.”

Multiple studies from Finland’s National Public Health Institute have also determined that daylight savings time disrupts sleep. In one study, ten healthy adult subjects were given special wrist activity clocks called accelerometers. This allowed the researchers to monitor the subjects’ rest-activity cycles for ten days during the transition period of daylight saving time.

This study was done for two consecutive years on the same people to confirm and calculate the results.

The research found that average sleep time was reduced by one hour and 14 minutes and average sleep efficiency went down by an average of 10%. This means of course that sleep disturbance went up by 10%, and the people lost an average of an hour and a quarter sleep due to the time change.

The same researchers conducted a study of 90 people between 20 and 40 years old between 2005 and 2006. Again they wore accelerometers. This study found that daylight saving time disrupted sleep by 54% during the fall transition and 37% during the spring transition. This was measured using motion during sleep and wakefulness — also called sleep fragmentation.

The researchers stated:

“Transitions out of and into daylight saving time enhance night-time restlessness and compromise the quality of sleep. They may thereby affect mood in a negative way and be a concern for individuals with mood disorder in particular.”

Other studies have showed similar results.

Learn how to adjust your body clock plus over 200 natural remedies to sleep better.

Why does daylight savings time continue despite proof that it increases sleep disturbance and reduces sleep time? And certainly there is the human cost — the confusion it creates, the cost in the massive efforts each year to adjust clocks back and forward.

Why? Something called stubbornness.


Hadlow NC, Brown S, Wardrop R, Henley D. The effects of season, daylight saving and time of sunrise on serum cortisol in a large population. Chronobiol Int. 2013 Oct 24.

Kantermann T, Juda M, Merrow M, Roenneberg T. The human circadian clock’s seasonal adjustment is disrupted by daylight saving time. Curr Biol. 2007 Nov 20;17(22):1996-2000.

Lahti TA, Leppämäki S, Lönnqvist J, Partonen T. Transition to daylight saving time reduces sleep duration plus sleep efficiency of the deprived sleep. Neurosci Lett. 2006 Oct 9;406(3):174-7.

Lahti TA, Leppämäki S, Lönnqvist J, Partonen T. Transitions into and out of daylight saving time compromise sleep and the rest-activity cycles. BMC Physiol. 2008 Feb 12;8:3. doi: 10.1186/1472-6793-8-3.