Permanent Daylight Saving Time Myths Lack Support: Karin Johnson, MD

Claims that permanent daylight saving time would increase physical activity and improve road safety are not supported by data, according to Karin Johnson, MD, a professor of neurology, sleep medicine specialist, spokesperson for the American Academy of Sleep Medicine, and former cochair of the Coalition for Permanent Standard Time.

Johnson said studies of light timing in the US show no rise in physical activity, as some people simply shift exercise from morning to evening. She added that circadian disruption and sleep loss are linked to less exercise. Even if activity did increase, she said, circadian effects on metabolism would still harm health. She cited about 20% higher obesity rates where mistimed clocks push sunrise and sunset an hour later, along with higher rates of cancer, diabetes, stroke, and heart attack.

The spring transition carries the greatest health and safety risks because people lose an hour of potential sleep and move to a more misaligned time, Johnson said. The impact is worse for those entering that week already sleep-deprived, including children, shift workers, patients with sleep apnea or other sleep disorders, and parents of young children. She advised getting adequate sleep in the days before the change and gradually shifting schedules to avoid adding more sleep loss.

Johnson also challenged the belief that permanent daylight saving time improves road safety because more people commute in the evening. She noted that a study of US locations on the western edge of a time zone, where sunrise and sunset come an hour later, found 20% more fatal car crashes. Permanent daylight saving time would also put the entire morning commute in darkness, she said, raising winter risks from icy roads and children waiting at school bus stops.

The last attempt at permanent daylight saving time, in 1974, ended partly because of 10 fatal crashes involving children at school buses in Florida in a single month, Johnson said.