Daylight Saving Time

Public opinion seems to be turning against Daylight Saving Time (DST), despite the fact that most of our clocks now change automatically, and despite the fact that most people don’t even seem to have a clear understanding of what DST does, and what its purpose is.

I’m not necessarily the biggest proponent of DST, but I do think it’s good in some ways, in some places. And if we’re going to abolish it, we should at least do it for good reasons. In this post, I’ll go over some of the things we should consider.

By DST, I mean the practice of seasonally changing our clocks back and forth. Or, depending on context, the summer phase of that cycle. Changing our clocks once and for all is not DST. There’s nothing wrong with calling it “year-round Daylight Saving Time”. I know what you mean. But “year-round DST” is not DST — it’s moving the time zone borders.

Note that DST typically lasts for more than half the year, but for brevity I’ll use imprecise phrases like “half the year”, or just “summer”/”winter”.

What is the point of DST?

The point of DST is to (effectively) move an hour of daylight from the early morning, to the late evening, on the presumption that it will be more useful there. This is only done during the part of the year when there is enough total daylight that we can afford to move it around.

Here’s a yearly daylight chart of New York City, if it did not observe DST. I’ve added dashed lines at 6 AM and 10 PM, showing when someone might get up in the morning, and go to bed at night.

[Charts generated by (edited by me)]

But New York does observe DST, so the actual chart looks like this:

With DST, the sunrise times vary less, and the sunset times vary more, over the course of a year.

Why is extended daylight in the evening more useful than in the morning?

Our mornings tend to be more structured than our evenings. Most people get up at about the same time every morning, and go through about the same routine. And most of those activities are indoors, so wouldn’t benefit greatly from sunlight.

Our evenings tend to be more diverse. We might work in the yard on Tuesday, stay inside on Wednesday, go shopping on Thursday, and attend an outdoor event on Friday. Most everyone will, at times, take advantage of late evening daylight if it’s present.

I think there are good practical and psychological reasons that our mornings are more structured, but the important thing is that they are, not why they are.

Your latitude matters

Near the equator, there is little seasonal change in the time of sunrise and sunset. Here’s a daylight chart for the Galapagos Islands, on the equator:

The only possible justification for DST in such a region would be to keep their clocks in sync with those of some other region.

In the extreme north and south latitudes, the usefulness of DST is diminished. Here’s a daylight chart for Anchorage, Alaska (which does observe DST):

There are windows of time in the spring and fall when DST does its job. But any way you slice it, in the winter you don’t have enough daylight, and in the summer you have more than you can use. DST doesn’t really help.

It’s the latitudes in the middle where DST is most useful.

Your longitude matters

Your longitude determines which time zones your region could reasonably choose to place itself in.

People near the western edge of a time zone experience daylight about one hour later than do people near the eastern edge of the same time zone. That’s a significant difference. But unless we greatly increase the number of time zones (which has its own drawbacks), it’s inevitable.

Because of this, DST has different consequences depending on one’s east-west position in their timezone. Those near the western edge get a lot of daylight in summer evenings even without DST, so they might prefer less DST. Those near the eastern edge might want DST to last even longer, or to spend some time on “double DST”.

An interesting idea is to double the number of time zones, and have only every other time zone observe DST. But again, that has drawbacks in increased complexity.

Your time zone matters

Whether you observe DST or not affects your sunrise/sunset times by one hour, for half the year.

Whether your region is in a particular time zone, versus an adjacent time zone, affects your sunrise/sunset times by one hour for the whole year.

So, which time zone you’re in is about twice as important (with respect to sunrise/sunset times) as whether you observe DST or not. Yet, in my experience, even people who live near a time zone border seem to care a lot more about whether their region observes DST, than about where to draw the time zone border. (Granted, the annoyance of changing our clocks must be taken into account.)

Astronomical considerations

If all our time zone borders were straight and evenly spaced, we could make it so that (non-DST) “12 noon” always occurs within about half an hour of the moment when the Sun is at its highest daily point in the sky. That really matters to some people. But personally, I don’t think it’s a worthwhile goal.

I think the main concern should be making sure “12 midnight”, and thus the transition from one date to the next, occurs at a time when most people are asleep. From this perspective, it might be better if “12 midnight” occurred a couple of hours later, relative to the Sun. But it’s good enough as it is. No need to upset the “astronomers” even more.

Why not change our schedules, instead of our clocks?

I really don’t see any way of making that happen. How would you legislate it? Even if you could, having every business and school change its hours twice a year would probably not simplify our lives, versus the other option of changing just one thing: our clocks.

What a mess, let’s just all use universal time!

What you’re saying is that if we also got rid of time zones, might that simplify things to the point where we can afford to have lots of different sets of business hours?

I’ve often seen it suggested that we all use universal time (UTC), but I have to wonder if the person suggesting it actually thought about it for more than 30 seconds. If we all used UTC, then for a certain large part of the world (basically, anywhere near the Pacific Ocean), 12 midnight would occur while people are awake, and out and about. The date, and the day of the week, would change in the middle of the active part of the day. That would be very inconvenient. I think that alone makes global UTC unworkable.

Pros and cons

The benefits of DST are small in number, but making better use of daylight is a pretty big benefit.

  • More daylight in the evening in the summer. Reduces the amount of daylight that is wasted by people sleeping through it.
  • Accomplishes the above without making mornings darker in the winter.
  • The economy: Capitalists will appreciate that people are more likely to go out and spend money, when there’s more daylight in the evening.

The drawbacks of DST are large in number, though most of them are pretty minor. Some examples:

  • Changing our clocks (physical and biological) twice a year causes a lot of problems, such as:
    • Some clocks have to be changed manually.
    • Accidents: No doubt, changing our clocks contributes to an increased number of accidents, in the days following the switch.
    • Effectively increases the number of time zones in the world, making long distance communication and travel somewhat more complicated. Even if every place in the world observed DST as consistently as possible, it would still increase the number of time zones, because the northern and southern hemispheres cannot observe DST at the same time of year.
  • Obviously, no aspect of DST is going to be good for every single person. People’s schedules and lifestyles are different. If you walk your dog early every morning, you probably don’t like DST.
  • There will be more morning commutes in the dark. There’s no getting around this as a drawback to DST. At latitudes where DST makes sense, it isn’t going to make anyone commute in the dark who doesn’t already commute in the dark for part of the year. But for many people, it increases the number of days in which they commute in the dark. Opponents of DST can reasonably warn about school children traveling to school in the dark. But keep in mind that “year round DST” could make this problem much worse than DST does.
  • At some latitudes, there’s enough light in the evening that it can mess with people’s circadian rhythms, so they don’t sleep as well.

Unclear issues:

  • Energy?: DST has in the past been promoted as a way to save energy (mainly on lighting and heating), as it better aligns our waking hours with daylight hours. But there is evidence that modern technology and associated cultural changes have pretty much eliminated this benefit.
  • Medical?: Studies may differ, but to the best of my understanding, while the switch to DST does lead to a few more medical events for a few days, this is balanced by a reduced number in the following days. It change the timing of medical events, but has little effect on whether they occur.


I’m sure I haven’t covered every relevant DST issue, but my point remains: Let’s at least try to understand DST before jumping on the “let’s abolish it” bandwagon.

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