It’s March, and for many North Americans, that means Daylight Savings Time  (DST) is near.  So why do the clocks spring forward – and how does Daylight Savings Time work?

In short, Daylight Savings Time is the practice of advancing clocks so that evenings have more daylight and mornings have less. Usually, clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn.  It is practiced mostly in North America, Europe, and several countries in South America.

Why do we do it?  An early goal of Daylight Savings Time was to save energy by reducing evening usage of incandescent lighting (formerly a primary use of electricity, especially in residential homes and large commercial buildings).  However, modern heating and cooling usage patterns differ greatly and research about how DST currently affects energy use is somewhat limited or contradictory.

In theory, people who want or need to maximize daylight could simply wake up earlier to take advantage of the sunlight… but this is unfortunately impractical due to the inflexibility of clock-based schedules.

Advantages and Disadvantages of Daylight Savings Time

Proponents of DST generally argue that it saves energy, promotes outdoor leisure activity in the evening, and is therefore good for physical and psychological health, reduces traffic accidents, reduces crime, or is good for business. Groups that tend to support DST are urban workers or professionals, retail businesses, outdoor sports enthusiasts and businesses, tourism operators, and others who benefit from increased light during the evening.

Opponents argue that actual energy savings are inconclusive, that DST can disrupt morning activities, and that the act of changing clocks twice a year is economically and socially disruptive and negates the benefits. Opponents also argue that it complicates timekeeping, disrupts business meetings, traveling, billing, record keeping, some medical devices, and sleep patterns.  Groups that tend to oppose Daylight Savings Time are farmers, transportation companies, and indoor/outdoor entertainment businesses.

Visualizing Daylight Savings Time

User “germanium” on visualized the sunrise and sunset times for each day throughout the year.  He explains how he did it below:

I wanted to see the effect of daylight saving time change on sunrise and sunset times. The data was taken from and is for Chicago. The figure shows that daylight saving time change (marked by the DLS lines) keeps the sunrise time pretty much constant throughout the whole year, while making the sunset time change a lot. The spread of sunrise times for the whole year as measured by the standard deviation is 42 minutes, while for sunset times is 1:30 hours.

How does daylight savings time work?

In any case, Daylight Savings Time is usually a welcomed change for people in countries that are in northern latitudes.  For those of us with long commutes home, driving in daylight and having a few hours to spend outside in the yard is well worth any disruption the clock shift might cause.