What Is Daylight Saving Time?


Biannual clock changes are common across the globe. Daylight saving time is the part of the year from spring to fall when clocks are set forward by one hour. Daylight saving time starts by “springing forward” in March. In November, the clock “falls back” one hour to switch from daylight saving time to standard time.

Compared to standard time, daylight saving time has less sunlight in the morning and more in the evening. Supporters of daylight saving time argue that this enables more time for business and outdoor activities later in the day.

On the other hand, sleep experts believe daylight saving time interferes with quality sleep and overall health by preventing the body’s internal clock from being synchronized with natural light from the sun.

In addition, the practice of changing clocks twice per year is disruptive and can cause lost sleep and reductions in health and safety. For this reason, many governments have proposed laws to abolish clock changes and implement either standard time or daylight saving time year-round.

We take a closer look at daylight saving time, including why countries observe it and how it may affect both sleep and health.

When Does Daylight Saving Time Begin?

In the United States, daylight saving time begins on the second Sunday of March. At 2 a.m. that Sunday, the clock is moved forward an hour to 3 a.m. This practice is referred to as springing forward. Once implemented, daylight saving time lasts for 238 days.

The start and end of daylight saving time is set by law in the United States. Other countries, though, may start daylight saving time on a different date.

When Does Daylight Saving Time End?

Daylight saving time ends in the fall on the first Sunday in November. On that day, at 2 a.m. local time the clock is set back to 1 a.m., so this transition is referred to as falling back. Once daylight saving time ends, standard time begins, which adds an extra hour of daylight to the morning.

How Does Daylight Saving Time Affect You?

The most immediate effect that most people feel from daylight saving time occurs because of the clock changes that shift between standard time and daylight saving time.

Studies have linked both time changes to sleep disruptions, especially when clocks are set forward in March. In addition to losing one hour of sleep on that Sunday, many people have sleep problems for the following week or longer. This can reduce alertness and has been associated with an increase in car crashes after switching to daylight saving time.

These transitions may affect people differently based on their sleep patterns. Early risers tend to have more problems with the fall clock change. People who are night owls with late bedtimes tend to have more difficulty with the spring clock change. People who already get insufficient sleep have challenges adjusting to both the fall and spring changes.

Additionally, daylight saving time can affect the body’s internal clock, known as its circadian rhythm. Daylight saving time does not change the length of the day, but it impacts which hours of the day have sunlight, which can disrupt circadian rhythm.

The effects of clock changes and daylight saving time on sleep and circadian rhythm can have negative consequences for effective thinking and overall health.

Sleep and Circadian Rhythm

Circadian rhythm is the body's 24-hour internal clock that regulates sleep and many other bodily processes. The biggest influence on circadian rhythm is exposure to light from the environment, allowing the body’s internal clock to synchronize with light from the sun.

Springing forward to daylight saving time is a sudden change to the hours of natural light, and that can interfere with circadian rhythm. Specifically, daylight saving time causes brighter evenings and darker mornings, which can misalign the body’s internal clock.

In addition, being exposed to light later in the day can make it harder to fall asleep at bedtime. Despite going to bed later, most people still have to go to work or school at the same time in the morning, and are more likely to get insufficient sleep as a result.

Memory and Alertness

The ongoing lack of sleep from the daylight saving time change can negatively affect thinking and mental performance.

Sleep loss makes it challenging to form new memories, impairs decision-making, and can increase the potential for mistakes at work. For students, a study linked insufficient nightly sleep to reduced academic achievement. Clock changes have also been associated with worse SAT outcomes for students.

A week of sleep loss can also adversely impact alertness. Daytime sleepiness can lead to health and safety risks, including drowsy driving. A study found that fatal car accidents increased by 6% in the week following the spring daylight saving transition. Sudden clock changes may also increase the risk of workplace injuries related to lack of sleep.

Mental and Physical Health

Insufficient sleep and misaligned circadian rhythm can have potentially serious consequences for mental and physical health.

Disrupted sleep can cause low mood and emotional distress. Sleep loss also increases appetite and the risk of obesity while worsening gastrointestinal disorders. When it persists over the long-term, sleep loss has been linked to an elevated risk of high blood pressure, diabetes, and cancer.

Additionally, an unhealthy circadian rhythm also has significant health impacts. Circadian rhythm disruptions are linked to an increased risk of multiple health issues, including obesity, diabetes, and cardiovascular diseases.

Why Do We Have Daylight Saving Time?

The history of daylight saving time is complex. The concept of adjusting clocks in the summer to extend daylight hours was introduced by Benjamin Franklin in a 1784 letter to the editor that suggested that a clock change could help reduce candle waste.

Actual adoption of daylight saving time was driven by wartime efforts to save energy. Germany became the first country to officially establish daylight saving time in an effort to conserve fuel during World War I. The U.S. Congress temporarily adopted daylight saving time at the federal level from 1918 to 1919. After the war ended, only some states continued to observe daylight saving time.

During World War II, the U.S. again used daylight saving time with the goal of saving energy. After the war, inconsistency emerged because each state and local government decided whether or not to observe daylight saving time. To remedy this, Congress passed the Uniform Time Act of 1966 that enacted the current system of switching between standard time and daylight saving time.

Permanent daylight saving was adopted again in January 1974 during a global oil crisis, but it was quickly repealed in October 1974 because of widespread public disapproval of the reduction in morning light.

The theory behind daylight saving time was that an extra hour of daylight in the evening would reduce electricity consumption. However, analysis of the evidence has found that in the present day there is little effect on energy use.

Advocates of daylight saving time also point to potential economic benefits from increased commercial activity in the evening. They argue that more evening light can reduce crime and make evening rush hour safer. In addition, some people prefer light later in the day so that they can do outdoor activities after work.

Will Daylight Saving Change in the Future?

A 2020 survey by the American Academy of Sleep Medicine found that 63% of Americans support eliminating biannual clock changes in favor of a fixed year-round time.

Numerous U.S. states have proposed eliminating clock changes. From 2015 to 2019, 29 states introduced laws supporting year-round daylight saving time. However, only Congress has the authority to amend the length of daylight saving time. In March 2022, the Senate passed the Sunshine Protection Act, which would make daylight saving time permanent nationwide. For the bill to become law, it would need to pass through the House of Representatives and get the approval of the President.

There is considerable debate about whether the U.S. should stay with its current system or abolish clock changes in favor of either permanent standard time or permanent daylight saving time. Considering the potential advantages and disadvantages of daylight saving time can help understand the key points in this debate.

Advantages of Daylight Saving Time

According to proponents of daylight saving time, some of its key benefits include:

  • Economic activity: Proponents argue that extended daylight may increase economic activity, such as shopping and tourism.
  • Physical activity: Extending daylight hours later in the evening may promote outdoor activities. One study found that walking and biking increased in the evening during daylight saving time.
  • Less crime: Some research has found a slight decrease in violent crime during daylight saving time because more public activity happens during daylight hours when crime is less common.

Disadvantages of Daylight Saving Time

Opponents of daylight saving time point out a number of drawbacks, many of which could be exacerbated if daylight saving time was made permanent.

  • Disrupted sleep: Sleep problems are an important public health issue related to daylight saving time. Pushing back clocks by an hour can throw off sleep timing and lead to later bedtimes that cut into total sleep. This can be harmful to adults as well as teenagers and school-aged children. Lack of morning light exposure can also disrupt circadian rhythm and sleep-wake cycles.
  • Risk of stroke and heart attack: The transition to daylight saving time is associated with increased strokes and heart attacks.
  • Circadian misalignment: Because circadian rhythm affects organs and cells throughout the body, there are a wide range of potential health consequences from the decreased morning light and the misalignment of the body’s internal clock. For example, studies have tied disruption of circadian rhythm to cardiovascular diseases, worsened immune function, diabetes, depression, and cancer.

Who Observes Daylight Saving Time?

Changing the clock between daylight saving time and standard time is done in many parts of the world including Australia, New Zealand, many western European countries, and the United Kingdom. But the dates of clock changes can differ between countries.

In the World

About 76 countries use daylight saving time, encompassing more than 1.6 billion people worldwide. Switching to daylight saving time in the summer is common in countries that are further north and have greater seasonal variation in total daylight hours. In contrast, most tropical countries near the equator do not observe daylight saving time since the amount of daylight is generally similar throughout the year.

In the U.S.

Not all U.S. states and territories observe daylight saving time. Hawaii and Arizona, except for the Navajo Nation, use permanent standard time and do not switch to daylight saving time. In addition, the U.S. territories, including the Northern Mariana Islands, American Samoa, Guam, Puerto Rico, and the Virgin Islands, don’t observe daylight saving time.

Tips for Adjusting to Daylight Saving Time

Clock changes can be inconvenient and bothersome, but some practical steps can help make them less disruptive.

  • Get good sleep beforehand: You may notice fewer impacts of sleep loss from the time change if you make sure to get enough sleep before the switch. Prioritizing sleep by going to bed and waking up at the same time each day can help improve sleep quality.
  • Simplify the transition: Set your clock forward before going to sleep, and try to go to bed around your normal bedtime. Staying up later than usual can exacerbate sleep loss.
  • Take a short nap: If you’re feeling sleepy during the day after the start of daylight saving time, try taking a 15 to 30 minute nap in the afternoon. However, you should avoid napping for more than 30 minutes since that can cause sleep problems at night and may cause you to wake up groggy.
  • Avoid late caffeine consumption: Try to avoid caffeine consumption for at least six hours before bedtime because caffeine can stay in your system and disrupt your sleep.
  • Get outside in the morning: Outdoor activities in morning daylight can help you feel alert during the day and help establish a healthy circadian rhythm. Staying physically active can also make it easier to sleep at night. Although artificial light exposure can have an impact, sunlight is much more powerful at affecting your energy level and internal clock.
  • Make your bedroom sleep-friendly: Try your best to make your bedroom comfortable. In addition, try to eliminate things that could disrupt your sleep such as bright lights or excess heat in your bedroom.

Frequently Asked Questions About Daylight Saving Time

Is It Daylight Savings Time or Daylight Saving Time?

Daylight savings time is a common misspelling. The accepted name is daylight saving time.

When Do the Clocks Turn Back?

In the U.S., daylight saving time ends on the first Sunday of November. The start and end times for daylight saving time may vary in other countries.


  • Circadian Rhythms: This page from the National Institute of General Medical Sciences offers background about the biology of circadian rhythms.
  • Check the Official Time: The National Institute of Standards and Technology shows the official time in time zones across the United States and assesses the accuracy of your device’s clock.
  • Drowsy Driving: This resource by the Department of Transportation provides facts about drowsy driving and tips to help prevent accidents tied to sleepiness.
  • Bedtime Habits for Infants and Children: Learn about sleep-related routines and how they can benefit young children in this resource from the National Library of Medicine.


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