But DST also creates problems, which can either be short-term ( acute) or long-term ( chronic). There are good sides to DST, such as coming home earlier (by the sun clock) from school or work and having more hours of daylight during our free time after work. How does Daylight Saving Time affect our health? Since the body clock follows the sun clock, these changes can affect our health. Instead of seeing DST as working according to one time zone to the east, one can also think of it as people’s body clocks being pushed further west within their time zone (or social clock). This means that people in Chicago have to work during the office hours of New York, and people in Berlin have the office hours of St. Essentially, they have to go to work in one time zone further to the east. People, who have to get up at 6 AM by the sun clock in winter, have to get up at 5 AM by the sun clock under DST despite the social clock showing 6 AM. During DST however, New York’s social clock shows noon when it is only 11 AM by the sun clock. For example, New York’s social clock closely matches the sun clock in winter during Standard Time: when the social clock says it’s noon, it is very close to midday, the sun’s highest point in the sky. However, DST disrupts the relationship between social and sun clocks. Although this turned out not to be true ( Aries and Newsham, 2008), DST is still kept to allow people to start work earlier by social clock time in summer. DST was thought to save energy because people would use less electric light in the evenings ( Kotchen and Grant, 2011 Mirza and Bergland, 2011). DST commonly lasts from spring to autumn and changes the social clock so that we simply start the social day one hour earlier relative to the sun clock. Many decades ago, politicians introduced Daylight Saving Time (DST). Thus, when two people wake up at 7 AM for work, the body clock of the person in the eastern edge may be set to 7 AM, but that of the person on the western edge may be set to 6 AM and the difference between the two clocks leads to health and safety problems. The best explanation for these findings is that the difference between our social clock – set by humans – and our body clock – set by the sun – increases towards the western edges of time zones. Several studies found that the further west people live within a time zone the more health problems they may experience and the shorter they live on average ( Borisenkov, 2011 Gu et al., 2017 VoPham et al., 2018). Body clocks are later relative to the social clock in winter, when nights are longer with earlier sunsets and later sunrises ( Hadlow et al., 2018, 2014 Hashizaki et al., 2018 Kantermann et al., 2007), and body clocks are earlier at the eastern edge of a time zone and become gradually later towards the western edge ( Borisenkov, 2011 Hadlow et al., 2018 Randler, 2008 Roenneberg et al., 2007). We live according to the same social clock time within a time zone, but as long as we still can see the natural day (through windows or on our way to or from work or school), our body clocks still follow more or less the time of the sun clock defined by the sun’s east-west path. Some countries (like China) have combined neighboring time zones to make their time zones bigger, so that the sun needs even longer than 1 hour to cross the time zone. Since country or state lines do not exactly match longitude lines, the time zone of a specific place may not be the same as expected by the longitude lines. This means that the sun rises and sets 4 minutes later every longitude line as it “travels” to the west, taking one hour across the 15 longitude lines that are generally one time zone. The zero line has been defined to go through Greenwich east of London in England. Longitudes are imaginary lines that run between the North and the South Pole and cut the earth into 360 even ‘apple slices’. It takes the sun 4 minutes to cross each longitude from east to west. Although we live according to the same clock time within a time zone (which we will call social clock), our body clocks still receive the timing signal from the sun defined by its east-west path ( sun clock). That is why the sun seems to travel from the east in the morning via its highest point near midday towards the west where it sets in the evening.
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