While more than 60% of countries around the world follow standard time all year, most of the United States (with the exception of Arizona, Hawaii, the Navajo Nation and the U.S. territories of American Samoa, Guam, the Northern Mariana Islands, Puerto Rico and the United States Virgin Islands) shifts between standard time and daylight saving time (DST) each year. Daylight saving time starts in the U.S. on the second Sunday in March and ends on the first Sunday in November. States that observe daylight saving time set their clocks forward one hour so that the evenings have an extra hour of daylight, and the mornings are darker when the days are shorter in the winter months.

However, many people, especially sleep physicians, question the impact that daylight saving time has on health.

“There has always been a debate in regard to daylight saving time,” says Phyllis C. Zee, MD, PhD, sleep medicine specialist and chief of Sleep Medicine in the Department of Neurology at Northwestern Medicine. “Science has evolved over the last decade to show the transition between standard time and DST is associated with bad health consequences. The big question on the table right now is, should it be permanent standard time or permanent DST?”

Dr. Zee shares why she is in favor of getting rid of daylight saving time and moving to year-round standard time.

What Is Daylight Saving Time?

DST is the time between March and November when most U.S. states, including Illinois, turn the clocks forward one hour. A few states, including Hawaii and Arizona, do not change from standard time.

In states that follow DST, on the day in March when people “spring forward,” many will lose one full hour of sleep. This one hour seems harmless, but it can wreak havoc on the body for days, and even months.

Daylight Saving Time: A Potential Barrier to Sleep

Sleep health depends on the balance between two processes:

1. Sleep homeostasis: the desire to sleep that builds from the moment you wake up through bedtime.
2. Circadian rhythm: the body’s natural 24-hour biological cycles, including the sleep-wake cycle that regulates the timing of sleep at night and wakefulness during the day.

Your circadian rhythm is set by the timing and amount of bright light exposure a person gets during the day. “With DST, between March and November, your body is exposed to less morning light and more evening light, which can throw off your circadian rhythm,” states Dr. Zee.

When your internal clock is out of sync with the sun’s clock, you can feel tired in the morning and awake in the evening. By throwing off your circadian rhythm, you also throw off your sleep homeostasis. As a result, your sleep health is at stake, along with a number of functions in your body, including your metabolism and mental health.

“If we adopt permanent standard time, our internal clocks will more likely be in sync with the rotation of the Earth, seasonal changes and the sun clock,” says Dr. Zee.

Daylight Saving Time: A Broader Health Hazard

“We know that sleep deprivation is bad for your physical and cognitive health,” notes Dr. Zee. She explains that the transition to daylight saving time can create short-term health problems — sleep issues, fatigue and changes in blood pressure — that feel like traveling to another time zone, or prolonged jet lag. “Late starters,” or those who wake up later in the morning, as well as teenagers, who tend to be night owls, are more vulnerable to these effects because they already sleep through more hours of natural morning light. DST can further throw off their circadian rhythms.

Additionally, daylight saving time can have long-term health effects. Studies show that DST is linked to:

1. Depression
2. Slowed metabolism
3. Weight gain
4. Cluster headaches

Daylight saving time has also been linked to increased risk of developing certain disorders, from cognitive and mental health issues to digestive and heart diseases. And, if you already have these conditions, DST can make them worse.

During the week after the shift to DST, research shows an associated rise in:

1. Cardiovascular disease, with a higher risk of heart attacks
2. Injuries, including a spike in fatal car accidents
3. Stroke rate
4. Mental health and cognitive issues
5. Digestive and immune-related diseases, such as colitis

Steps to Saving Sleep During Daylight Saving Time

Leading up to and during daylight saving time, protect your circadian rhythm and your health by:

1. Keeping a sleep routine: Aim for seven to nine hours of sleep each night. The night before DST starts, go to bed one hour earlier than normal to prepare. This is an especially important technique for parents with young children.
2. Chasing the morning light: For one to two days leading up to the switch to DST, get outside in the morning. More natural morning light can help preserve your circadian rhythm. Throughout DST, spend as much time outside in the mornings as you can. Instead of looking at your phone in the morning, try to take in as much sunlight as possible.
3. Eliminating sleep disturbances: Avoid excess amounts of caffeine, alcohol and blue light exposure (from screens) one to two hours before bedtime.
4. Exercising in the morning: Activity raises your body temperature, which can increase your wakefulness and help reset your internal clock.