Your turn
Each lab is one step from the video. Drag, slide and switch, and watch the daylight move.
Each lab isolates one idea. Sunrise and sunset come from a simplified model (see the note in the sunlight lab).
Tap and slide, and watch what happens.
Drag the clock hand through a March night. What happens after 1:59?
At 2:00 a.m. local time the clock jumps to 3:00 (March) or back to 1:00 (November). Drag through both nights.
Turn the clock hand. Watch it jump!
Flip daylight saving time on and off. Does the Sun move, or the clock?
Same solar day, clock shifted +1 h. Compare the daylight that falls in waking hours.
Tap the button. Does more sunshine land in the evening?
Rounded June times for U.S. mid-latitudes. The awake window is an example day.
Tap each person to see what they really proposed.
Franklin's satire, Hudson's paper, Willett's campaign: which one actually proposed moving the clocks?
Drag along the timeline from 1900 to 1975. When did the U.S. use daylight saving time?
Drag through 1907–1966: adoption, repeal, "War Time" and standardisation.
Pick a place, slide through the year and flip the clocks. Where does daylight saving help the most?
The yellow band is daylight over the year. Compare how much morning light there is to shift at 0° and at 60°N.
Simplified model: day length from Earth's 23.44° tilt, the Sun highest at exactly 12:00 standard time, no atmosphere. Real towns can be half an hour or more off.
Tap each study. Did it save electricity, or use more?
Three estimates, three answers: compare the size and sign of each effect.
Once a year, on a spring night, the clocks jump ahead one hour. In the fall, they go back again. This is called daylight saving time.
In summer, the Sun comes up very early in the morning. Most people are still asleep! The sunshine lands on closed curtains.
When we move the clocks ahead, the Sun does not change at all. But now more sunshine is left for the evening, when you can play outside.
Long ago, during a big war, coal was very precious. People burned it to make light. Light evenings meant fewer lamps and less coal.
Near the middle of the Earth, days are about the same length all year. So lots of places, like Hawaii, never move their clocks.
Daylight saving time (many people say "daylight savings") exists to move an hour of summer daylight from the early morning, when most people are asleep, to the evening, when most people are awake. Clocks "spring forward" one hour in spring and "fall back" in the fall.1 It was first used by whole countries in 1916, to save coal that would otherwise be burned for lighting.59
The key idea: daylight saving time doesn't create any extra daylight. The Sun rises and sets exactly as it would anyway. We just relabel the hours on our clocks so the light lands at a more useful time of day.1
Daylight saving time, or DST, is the part of the year when clocks are set one hour ahead of standard time. In the United States the switch happens at 2 a.m. local time: in March, clocks jump from 1:59 a.m. straight to 3:00 a.m., and in November they go from 1:59 a.m. back to 1:00 a.m.1 Since 2007, U.S. daylight saving time runs from the second Sunday in March to the first Sunday in November, about 238 days, or roughly 65% of the year.113
In the United Kingdom and much of Europe it's called summer time.12 The official name in the U.S. is "daylight saving time", without an s, but "daylight savings time" is so common that encyclopedias list it as another name.12
Picture a June day in much of the United States. By standard time, the Sun rises around 4:30 a.m. and sets around 7:30 p.m.12 Most people sleep through that first bright hour or two. After clocks spring forward, the same sunrise happens at 5:30 a.m. and sunset at 8:30 p.m. by the clock. Nothing in the sky changed, but an hour of light has moved into the evening.
| June day (example) | Standard time | Daylight saving time |
|---|---|---|
| Sunrise | about 4:30 a.m. | about 5:30 a.m. |
| Sunset | about 7:30 p.m. | about 8:30 p.m. |
| Hours of daylight | about 15 | about 15 (the same!) |
Rounded times for U.S. mid-latitudes around the June solstice; your town will differ a little.1012
You'll often read that Benjamin Franklin invented daylight saving time. He didn't. In 1784, while living in Paris, he wrote a playful letter suggesting Parisians could save candles by getting up at sunrise. It was satire, and it never proposed changing the clocks.512
The first real proposal came from George Vernon Hudson, a New Zealand entomologist (insect scientist). His shift work left him free time to collect insects, and he wanted more daylight after work. In 1895 he presented a paper to the Wellington Philosophical Society proposing to move clocks two hours ahead in summer.12
In England, builder William Willett noticed on an early-morning ride how many Londoners were still asleep on a bright summer morning.12 In 1907 he began campaigning to set the clock ahead by 80 minutes in four 20-minute steps during April, and reverse it in September. Parliament rejected a bill in 1909.5
The idea finally became law in 1916, when Germany and Austria-Hungary adopted it during World War I to save coal: lighter evenings meant fewer lamps burning fuel.912 Britain and many other countries followed, and the United States passed its first daylight saving law in March 1918.45
The U.S. repealed its national law in 1919.34 Year-round "War Time" returned from 1942 to 1945.5 In between and afterwards, towns and states largely chose for themselves, until the Uniform Time Act of 1966 set standard dates for every place that uses daylight saving time.912
Daylight saving only helps where summer days are much longer than winter days, and that depends on how far you are from the equator. Earth's tilt makes day length swing through the year, and the swing grows the farther you go from the equator (see what causes the seasons).10
In the United States, Hawaii and most of Arizona don't observe daylight saving time (the Navajo Nation in Arizona does), and neither do American Samoa, Guam, the Northern Mariana Islands, Puerto Rico or the U.S. Virgin Islands. Under the Uniform Time Act, a state may opt out by state law, but if it uses DST it must follow the national dates.12
About the "Does it help everywhere?" lab: it uses a simplified model. It computes day length from Earth's 23.44° tilt and assumes the Sun is highest at exactly 12:00 standard time, with no atmosphere. Real towns can be half an hour or more off, depending on where they sit in their time zone, so treat the times as a rule of thumb.
Saving fuel was the original reason, and it's still given as a reason today. The U.S. extended daylight saving time in 2007 "in the interest of reducing energy consumption".1 But when scientists measure it, the effect is small, and not always a saving:
| Study | What it found |
|---|---|
| U.S. Department of Energy report to Congress (2008) | The extra four weeks saved about 1.3 terawatt-hours, about 0.03% of U.S. electricity for the year.6 |
| Review of 44 studies (Havranek and colleagues, 2018) | On average about 0.34% less electricity on daylight-saving days; bigger savings far from the equator.8 |
| Indiana households (Kotchen and Grant, 2011) | Homes used about 1% more electricity, because extra heating and cooling outweighed the saved lighting.7 |
Why so small? Lighting is only part of the picture. A lighter evening can mean less lamp use but more air conditioning, and a darker morning can mean more heating, so the effects partly cancel out.7
The American Academy of Sleep Medicine says that moving into or out of daylight saving time disrupts sleep for about 5 to 7 days, and advises getting plenty of sleep around the change (at least 9 hours a night for kids aged 6 to 12).9 Going to bed a little earlier for a few nights before the spring switch can help. (Why does a lost hour matter so much? See why sleep is important.)
People disagree about whether light evenings or light mornings are better, and about the costs of switching twice a year. Debates continue in many places, and any change needs a new law.29 Meanwhile, your phone and computer handle the switch for you: official time services broadcast one universal time, and your device's software adds your time zone and daylight saving rules.1 (Precise shared time matters for more than alarms: see how GPS works.)
Daylight saving time moves clocks one hour ahead in spring so that more of the summer's daylight falls in the evening, when most people are awake, instead of the early morning, when most people are asleep. It was first used nationally in 1916 to save coal for lighting, and today many people simply like the long, light summer evenings.
The first national daylight saving time began in Germany and Austria-Hungary in 1916, during World War I, to save coal by cutting the need for lamps in the evening. Britain, the United States and many other countries soon followed. The idea itself came earlier, from George Hudson in New Zealand (1895) and William Willett in England (1907).
No. That's a common myth. Farmers were some of its strongest opponents, because farm work such as milking follows the Sun, not the clock. In the United States, farm groups helped get the first national daylight saving law repealed in 1919.
No. Many countries use it, including most of Europe, where it's called summer time, and New Zealand, where George Hudson first proposed the idea. Many countries near the equator never use it, because their days stay close to 12 hours long all year, so there is little daylight to move.
Hawaii and most of Arizona don't change their clocks. The Navajo Nation in Arizona does. American Samoa, Guam, the Northern Mariana Islands, Puerto Rico and the U.S. Virgin Islands also stay on standard time all year. Under the Uniform Time Act, a state can choose to skip daylight saving time, but if it uses it, it must follow the national dates.
A little, at most, and not everywhere. A 2008 U.S. Department of Energy report found that the 4 extra weeks of daylight saving added in 2007 saved about 0.03% of the country's electricity for the year. A review of 44 studies found an average saving of 0.34% on daylight-saving days, but a study of Indiana homes found they used about 1% more, because extra heating and cooling cancelled out the saved lighting.
Changing it takes a law, and people disagree about what's best. Some prefer light evenings, others prefer light mornings, and sleep doctors point out that switching clocks disturbs sleep for several days. Debates about keeping, ending or changing the switch continue in many places.