Santa Barbara, Calif.

WHY do we  along with 75 other countries  alternate between standard time and daylight time? Although many people believe it has an agricultural provenance, daylight time has always been a policy meant to save energy. As Benjamin Franklin argued, if people moved up their summer schedules by an hour, they could live by “sunshine rather than candles” in the evenings.

Energy conservation was the motivation for daylight time during World Wars I and II and the oil embargo of the 1970s, and it remains so today  even though there has been little scientific evidence to suggest daylight time actually helps us cut back on electricity use.

Recently, however, we were able to conduct a study in Indiana, where daylight time was instituted statewide only in 2006. Before that year, daylight time was in effect in just a handful of counties. This change of policy offered a unique, natural experiment to measure the overall effect on residential electricity consumption. We could compare the amount of energy used by households in the late-adopting counties during the two years before they switched to daylight time with the amounts they used during the year afterward  while using counties that always practiced daylight time as a control group.

We found that daylight time caused a 1 percent overall increase in residential electricity use, though the effect varied from month to month. The greatest increase occurred in late summer and early fall, when electricity use rose by 2 percent to 4 percent.