This past Sunday, almost all Americans took part in the familiar ritual of changing the time on their clocks. Every year, we “spring ahead” in the spring (though it often still feels like winter) and “fall back” in the fall.

While the extra hour of sleep in the fall can feel like a small gift, the transition isn’t always easy. The earlier sunsets can make evenings feel shorter and darker, often leaving people feeling more tired or out of sync as daylight fades sooner. Each year, as we adjust to the change, many find themselves wondering why we continue to shift between standard time and daylight time. As with many things, there’s a law behind the entire process.

Daylight Saving Time (DST) was first introduced in the United States in 1918. The system has undergone multiple adjustments since then, with the most significant federal framework established under the Uniform Time Act of 1966, Public 89-387. Over the decades, Congress has amended the policy several times — primarily to modify the start and end dates of DST — culminating in the Energy Policy Act of 2005, Public Law 109-58 which set the current schedule. The United States Code currently outlines time in Title 15 §§ 260-67.

Oversight of Daylight Saving Time and the nation’s time zones falls under the U.S. Department of Transportation. Today, every state except Hawaii and most of Arizona (with the exception of the Navajo Nation) observes DST. The U.S. territories — American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the U.S. Virgin Islands — also do not participate. Under federal law, states may choose to opt out of observing Daylight Saving Time through legislative action. However, they cannot choose to remain on daylight saving time permanently without an act of Congress.

Over the past seven years, 19 states have passed legislation supporting year-round Daylight Saving Time, contingent on federal approval — and, in some cases, similar action by neighboring states. However, because current federal law prohibits permanent DST, Congress must act before any state can officially make the change.

According to the National Conference of State Legislatures, in 2025, at least 30 states considered or are considering DST legislation. For more information on Daylight Savings Time, check out the 2020 report from Congressional Research Service, which also provides an overview of several studies examining the effects of Daylight Saving Time on safety, energy consumption, and public health.