Daylight Saving Time, Explained

Twice a year, around 1.5 billion people change their clocks. The idea is older than the lightbulb, the science behind it is shaky, and most of the world has never bothered with it at all. Here's how it works and why we keep arguing about it.

What daylight saving time actually does

Daylight Saving Time (DST) is a seasonal one-hour adjustment to civil time. In spring, clocks "spring forward" — 2:00 AM becomes 3:00 AM, and the day loses an hour. In autumn, clocks "fall back" — 2:00 AM becomes 1:00 AM, and the day gains one back.

The goal is to shift an hour of daylight from the early morning, when most people are still in bed, to the evening, when more people are awake to use it. Your sleep schedule shifts, but the sun does not.

Where the idea came from

The first serious proposal came from a New Zealand entomologist named George Hudson in 1895. He worked shift hours, collected insects in his free time, and wanted more daylight after work. His paper to the Wellington Philosophical Society proposed a two-hour spring shift. It was politely ignored.

Britain's William Willett independently proposed a similar scheme in 1907, frustrated that Londoners "wasted" daylight every summer morning. He campaigned tirelessly for it until his death in 1915 — one year before the first country actually adopted it.

That country was Germany, in April 1916. It was wartime, coal was rationed, and the government wanted to reduce evening lighting demand. The UK followed three weeks later. The US adopted DST in 1918. After the war ended, most countries scrapped it. It was reintroduced widely during World War II for the same energy-saving rationale.

Who observes DST today

Despite the impression that the whole world flips its clocks twice a year, DST is the global minority. Roughly 70 countries observe some form of it. The vast majority of the world's population — including all of China, India, Japan, most of Africa, and most of South America — does not.

Where it is observed:

• The United States (except Hawaii, most of Arizona, and the territories): begins second Sunday in March, ends first Sunday in November.
• Canada: same dates as the US, except Saskatchewan and parts of Quebec, BC, and Ontario.
• The European Union and UK: begins last Sunday in March, ends last Sunday in October.
• Mexico: abolished DST nationally in 2022, except for a strip of border municipalities that align with the US.
• Australia: only the southeastern states (NSW, Victoria, Tasmania, ACT, South Australia). Begins first Sunday in October, ends first Sunday in April — the southern-hemisphere mirror image.
• New Zealand, Chile, Paraguay, and a handful of others.

Where it is not observed:

• China — the entire country runs on a single time zone (UTC+8) without DST
• India, Pakistan, Bangladesh, Sri Lanka, Nepal
• Japan, South Korea, Singapore, Vietnam, Indonesia, Philippines, Thailand
• Most of Africa
• Russia (abolished in 2011)
• Brazil (abolished in 2019)
• Iceland, Belarus, Turkey

Does it actually save energy?

This was the original justification, and modern evidence is decidedly mixed. Studies of the 2007 US extension of DST and of Indiana's adoption of DST have generally found that any electricity savings on lighting are offset by increased use of air conditioning in the evening and heating in the morning. The net effect is roughly zero, possibly slightly negative.

The energy argument made more sense in 1916, when most evening electricity went to incandescent lightbulbs and home heating barely existed in the modern sense. It makes much less sense today.

The case for keeping it

• More evening daylight for outdoor recreation, retail, and dining — industries that consistently report higher revenue during DST months.
• Lower evening crime rates, particularly robbery, in studies that have looked at the effect of the spring transition.
• Fewer pedestrian fatalities in the evening commute, since drivers are more likely to be travelling in daylight.

The case against

• Health effects from disrupted circadian rhythms — a measurable spike in heart attacks, strokes, and workplace accidents in the days following the spring transition.
• Sleep loss, particularly affecting children, shift workers, and people with existing sleep disorders.
• Disrupted morning light in winter — when DST extends into the dark months, school children commute in the dark, which sleep researchers consistently flag as a problem.
• Computer system bugs. The twice-a-year shift causes a steady stream of scheduling failures, billing errors, and missed alarms across software that doesn't handle the transition cleanly.

The current debate

In 2019, the European Parliament voted to abolish the seasonal clock change, leaving each member state free to choose either permanent winter time or permanent summer time. Implementation has stalled, and most of the EU still observes DST as of 2026.

In the United States, the Sunshine Protection Act — which would make DST permanent year-round — has passed the Senate twice but has not cleared the House. Sleep researchers and major medical associations have lined up against permanent DST and in favour of permanent standard time, arguing that morning sunlight matters more for human health than evening sunlight.

Most observers expect the seasonal shift to disappear within the next decade. Whether countries land on permanent standard time or permanent summer time is the real fight.

The bottom line

Daylight saving time is a 110-year-old wartime energy policy that mostly outlived the problem it was designed to solve. It still has its defenders — and its real benefits in the evening — but the consensus among researchers is that the twice-a-year shift causes more harm than good. Expect it to be phased out in most observing countries within your lifetime.