Daylight Saving Time (DST) is a time adjustment convention based on the actual daylight hours of the day. In countries that observe this convention, clocks are set forward by one hour from around the end of March (mid-Spring) to the beginning of November (late Fall). The rest of the year, clocks revert to standard time.
Why Adjust the Clocks?
When discussing time, people often think in two concepts. The first is time measured by daylight, dividing the day into “daytime” (the period of light) and “nighttime”. The second concept divides the day into equal time segments, which most people currently understand as 24 hours in a day.
However, due to the Earth’s axial tilt of 23.5 degrees, the duration of daylight varies across different seasons. In Summer, daytime is longer than nighttime, while in Winter, the opposite occurs. This phenomenon is more pronounced in locations closer to the poles.
In Summer, the sun rises earlier, and the days are longer. (Photo: National Geographic)
Humans have recognized this phenomenon for centuries. Some societies, like ancient Rome, divided daytime evenly, with varying lengths depending on the season. For instance, their water clocks indicated that during the Winter Solstice (mid-Winter), each hour of daylight lasted only about 44 minutes in today’s terms, while during the Summer Solstice (mid-Summer), each hour lasted up to 75 minutes.
According to DST, clocks are adjusted forward by one hour for most of the year. (Photo: iStockPhoto)
Over time, the division of a day into 24 hours became widespread, characterized by the fact that the actual duration of each hour remains constant throughout the year. However, this division makes the use of daylight less efficient.
In society, working or studying hours are typically strictly regulated according to the 24-hour day convention. Meanwhile, human activity often follows the natural light cycle: waking up and feeling energized when it’s light and going to sleep when it’s dark. Thus, if Winter working hours are applied in Summer, people would wake up only a few hours after sunrise, reducing work efficiency.
Additionally, people sleeping later in the Summer leads to increased electricity consumption for lighting, resulting in energy waste. This concern gained more attention in the latter half of the last century when energy shortages became an issue for many countries. This is why the Daylight Saving Time convention was proposed and is officially known as “Daylight Saving Time” (DST).
When Do We Change the Clocks?
Daylight Saving Time is commonly observed from the end of March to the end of October, spanning more than half a year. However, the specific dates for implementation vary by country, and not all countries follow this convention.
Map of regions currently observing DST (green) and those not (red and orange). (Photo: Wikipedia)
In Asia, only a few countries apply DST, such as Iran, Israel, and Turkey. Vietnam and major countries like China, Japan, and South Korea do not observe this convention. Countries that do typically start observing DST from the last Friday of March until the last Friday or Sunday of October.
In Europe, most countries, except for Russia, Iceland, and Belarus, observe Daylight Saving Time. DST begins at 1 AM (GMT) on the last Sunday of March and ends at 1 AM on the last Sunday of October.
In the Americas, three countries in North and Central America (Canada, the USA, and Mexico) and a few in South America observe DST. In the USA, some states do not participate in this convention. In most other states, clocks are adjusted at 2 AM on the second Sunday of March and reverted on the first Sunday of November.
What Are the Effects of Changing the Clocks?
From the examples above, it is evident that the timing of clock adjustments under DST varies widely and is complex. The adoption of this convention also significantly impacts human health, requiring an adjustment period, and has sparked much debate regarding its implementation.
Many studies have shown that the early clock adjustment at the end of March can negatively affect human health. Specifically, changing the time can disrupt sleep patterns, and it may take several days to readjust. A study in 2008 indicated that during the first week after the time change, the incidence of heart attacks spikes.
Does DST really save energy? (Photo: ABC News)
Another goal of DST is energy conservation, which is not entirely convincing. Although people may go to bed earlier in the Summer, they still go to work daily, thus still requiring energy for lighting during working hours. Furthermore, longer days lead to people spending more time outdoors, which can increase fuel or electricity costs for transportation.
During DST, people need to adjust their activities to fit the new time
Modern computers and smartphones often have features for automatic time adjustment based on DST; however, this feature does not always function correctly. Besides a recent bug in iOS 7, Apple faced issues with DST in 2010 when a bug in iOS 4 caused alarms to go off an hour late after the time change. The change in time also affects time-sensitive activities like flight schedules and showtimes, leading to confusion and disruptions.
In Vietnam, while DST is not observed, its effects can still be noticed when following international events or working with foreign individuals. Particularly, if you are a football fan, you will easily notice the impact of DST.
For most of the season, football fans will have to stay up late to follow exciting matches
Matches in the UEFA Champions League typically start at the same time, 7:45 PM GMT. At the start of the season, since Europe observes the time change while Vietnam does not, these early matches occur at 1:45 AM Vietnam time.
However, from November through the end of March, which is the majority of the football season, the time is adjusted, and the UEFA Champions League matches start at 2:45 AM, concluding quite late (around 5 AM), affecting the daily schedules and routines of Vietnamese people.
