Daylight-Losing Time

27 October 2010

The temptation of Daylight-Savings Time is easy to explain. If one times the activities that need light to occur as little as possible during times when light would have to be artificially generated, then one saves resources. As one moves away from the Winter Solstice, the time between sunrise and sunset lengthens, and the amount of natural light available in the mornings increases. We used time zones so that clocks are synchronized over reasonably large areas, rather than just long lines of longitude. And re-setting clocks is less failure-prone and otherwise cheaper than changing time-tables.

Unfortunately, Daylight-Savings Time doesn't really work. Worse, it kills people.

In the contiguous United States (the lower 48), the difference between the time of sunrise at the summer solstice and at the winter solstice is about two hours, and it's not as if the change takes place all at once. When an hour is added to the clocks, activities that were beginning at about sunrise are immediately beginning about an hour before sunrise; it takes more than a month for the seasonal change to catch-up to the clock change, and later in the year, as sunrise again has begun to take place later, there will be another month during which the seasonal difference is less than the clock difference. And the clocks through-out each time-zone, all the way to its western border, are typically being kept in synchronization with those on the eastern border; with time-zones being about an hour wide, activities that were taking place up to an hour after sunrise on the eastern border are taking place at or before sunrise elsewhere in the time zone. So we shouldn't be terribly shocked that statistically studies haven't been able to tease-out much-if-any actual savings associated with Daylight-Savings Time.

Meanwhile, it has been observed that, as the nation goes on or off Daylight-Savings Time, there is an increase in automobiles hitting pedestrians. That's because drivers adjust imperfectly to the apparent sudden change in how dark it is in the morning or in the evening. They are driving in the morning or in the evening as if it is lighter than it is, and the fact that they are driving as if it is darker than it is at the other end of the day doesn't offset the effects (because the marginal effect of caution is diminishing). When Daylight-Savings Time is begun, there an addition element of people being poorly rested; the effect is not much off-set by people being better rested when Standard Time is resumed.

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