Daylight Saving Time: When Do the Clocks Change and Why

Early Sunday morning, time will repeat itself, as daylight saving time ends in the U.S. and Canada. At 2 a.m., clocks will jump back an hour, bringing more light to your morning commute but meaning the sun will likely set before you leave the office. We'll remain in what's laughably called "standard time" for just four months, until March 11, 2018, when the clocks move forward and daylight saving time begins again.

Those dates are actually written into the U.S. Code, which defines the "advancement of time" as beginning the second Sunday of March and ending the first Sunday of November. It includes every American except for those living in Arizona and Hawaii, which are exempt and choose to ignore daylight saving time, like most of Asia and Africa and large chunks of Australia and South America.

Daylight saving time has many vocal detractors (including your pets, who have absolutely no idea what you're doing each time change). But it has been an American institution for almost a century, since it was originally instated during World War I as a cost-saving measure by saving energy from lights in the evening—although it's unclear whether shifting the clocks to optimize daylight hours really does save money.

What behavioral scientists do know is that Americans drive more when the sun is out—which is why gas companies are among the strongest supporters of daylight saving. (Farmers, who are often blamed for the innovation, actually hate it.) And while the extra sunlight may cheer us up a little, the disruption to sleep cycles causes enough harm to outweigh it.

It can be easy to confuse daylight saving time with time zones, but those are grounded in a much more basic astronomical reality: Day and night is governed by the Earth spinning on its axis, so that sunlight gradually visits and then abandons each north-south swath of the globe. Each time zone is designed to cover about one twenty-fourth of the circumference of the Earth, although their borders zig and zag to keep political entities together. That makes it so that the sun is at its highest sometime around noon.

But Americans don't tend to split their day evenly around the noon marker; instead we tend to put school or work first, then take the evening as leisure time. Daylight saving is designed to reshuffle the sunlit hours to better match that societal schedule. And at this point, it's so deeply ingrained that no one has quite managed to overthrow the practice.