How does dst affect time zones

Nowadays, most changes between winter- and summer time happen at the same time - even across national borders. Especially in the earlier years this was still a wild mess without established rules. Additionally, not all regions of a country have to change their timezones at the same time - or even at all. These decisions are often based on goverments in federal states. The entraining process shows individual variations in the relationship between the body clock and the light—dark cycle e.

The term chronotype has some times been associated with a relatively stable personality trait or with so-called daily preferences Horne and Ostberg, , whereas we use it here as the actual phase i.

Phase of entrainment can be assessed in different ways both subjectively and objectively. An example for the former is the Munich ChronoType Questionnaire MCTQ that uses subjectively assessed sleep and wake times, specifically the midpoint between sleep onset and offset Roenneberg et al. Historically in humans and other seasonally reproducing animals, the body clock responded to the number of hours of daylight that corresponded internally to the number of hours the internal circadian signal melatonin is secreted Bittman et al.

The nocturnal duration of melatonin expanded in winter and was compressed in summer. With the invention of artificial light, this zeitgeber has been drastically weakened since we now predominantly spend most of our days in buildings, thereby rarely getting full sunlight, and we switch on artificial light after sunset, thereby only being exposed to darkness but not necessarily total darkness when we sleep.

One consequence of this weakening zeitgeber strength — on the annual level — is that seasonal rhythms like human reproduction have become weaker and possibly only socially driven Roenneberg, This zeitgeber-strength-dependent change in entrainment can be predicted from circadian formalisms Roenneberg et al.

As a consequence, the distribution of chronotypes has become later and much wider than in pre-industrialized conditions Roenneberg et al. When the social clock does not follow the large delays of the body clock , significant discrepancies between these two clocks arise; this so-called circadian misalignment can be assessed for some situations by calculating social jetlag SJL , which is the difference between sleep-timing on work and work-free days Wittmann et al. We discuss the importance of SJL later in this paper.

Different chronotypes can therefore be translated to or conceived in a thought experiment as living in different longitudinal locations. According to this translation, every chronotype virtually lives in its own chronobiological time zone that — unfortunately in many cases — is different from where the individual actually lives.

While all people from Chicago physically live in Chicago [yellow arrows in Figure 1 , which was first presented at the Sapporo Symposium Roenneberg et al. The discrepancy between the physical time zone and the chronobiological time zone was small when we lived without artificial light as shown in Figure 1A.

Figure 1. A map of the continental United States showing the actual time zones color-coded in the background, i. The political time zones adopted by some areas and states of the United States differ from the physical time zones.

A Pre-industrial without electric light blue: Eastern; green: Central; yellow: Mountain; red: Pacific ; B in post-industrial times with electric light but under Standard Time; C as B , but under Daylight Saving Time, which amounts to a reassignment of the political time zones [note the color change compared to A and B ].

Note that the owners of these body clocks have to work in Chicago. This situation becomes even worse when social clocks are switched to DST, which means that the people of Chicago now have to work according to the sun clock in Nova Scotia, Canada which is 1 h earlier pink, illustrated in Figure 1C.

In the pre-industrial era, humans slept mostly between 8 p. As one can see in Figure 1A , DST adds an hour to the discrepancy between the social and the body clock thereby fueling the battle between biological and social time and increasing SJL. Unfortunately, opinions expressed in poll answers and potential decisions based on such opinions may not be based on scientific evidence.

In addition, both the lay public and scientists use language in relationship with DST that invites prejudice. Importantly, days are not becoming additionally longer and the sun does not set additionally later because of DST, we simply come home earlier in reference to the sun clock because we start work or school earlier in reference to the sun clock.

DST changes are not comparable with time changes after transmeridian flight known as jet lag because we stay where we are instead of exposing our body clocks to the new light—dark cycles of our travel destination. Table 1. However, when the switch to DST occurs, they are getting up before the sun again, throwing the relationship between sunrise and get-up back by three weeks or more depending on latitude in their seasonal trajectory.

The question is, however, what happens if this mismatch of 1 h is maintained throughout the DST period see below? In September , two sleep researchers from the University of Salzburg claimed in an interview that there is no hard scientific evidence against perennial DST and that the risks would be negligible Press Release University of Salzburg.

This press release contained several statements that echo wide-spread incorrect beliefs and is therefore an excellent substrate for clarifying fallacies Table 1.

The above statement by the Salzburg researchers is misleading in two ways. First, the spring and the autumn effect do not balance each other out on the individual level and the higher risk in spring is avoidable by abolishing DST. Second, the paper by Janszky and Ljung show a decrease in risk on the days immediately following the autumn release from DST. If physiology had fully adapted to DST and if the decreased cardiac risk was not only from the extra hour of sleep the night DST ends, this decrease should not occur.

Thus, the results of this paper suggest that the risk for myocardial infarction was elevated throughout DST.

In contrast, entrainment by external temperature changes is most likely not relevant in mammals Buhr et al. A paper investigating sleep in pre-industrial societies Yetish et al. But cold morning temperatures per se do not entrain the human clock and are thus more comparable to an alarm clock than to a zeitgeber. The strong blue components … are the true robbers of sleep.

The second half of the statement, however, is missing the point: the combination of nighttime light exposure and DST is far worse than nighttime night exposure alone. The nighttime light exposures delay the body clock in relation to the sun clock Roenneberg et al. Thus, in DST, the two effects additively i. If you look at the results carefully, the reported effects are very small.

When we do this, we see that permanent DST would increase the average SJL by more than half an hour which may be statistically small but is biologically large. Russia was not the only country to try and then abandon permanent DST.

A revealing review of the political considerations within the United States of the DST laws is available in Gray and Jenkins ; farmers, parents of children who waited for school buses in the dark, and people in higher latitudes were especially vocal against permanent DST. In the United States, minimal or no energy savings were observed as noted above , and there were some increases in traffic accidents in the morning.

These positive effects may go beyond subjective feelings. A study has shown for example that activity increases with longer evening daylight Goodman et al. Interestingly these results of the above study were culture-specific: a significant increase was mainly observed in Europe and to some extent in Australia, while no significant effects or even slightly negative effects were seen in the United States and Brazil. The first days after the DST change in spring show acute effects: sleep is shortened Barnes and Wagner, , adolescents are sleepier during the day Schneider and Randler, , general accidents and visits to the emergency room increase Ferrazzi et al.

On the Monday after the DST transition, the known stock market weekend effect i. Reports have been contradictory on the acute effects of DST on traffic accidents Carey and Sarma, Some find an increase after transitions to and from DST Hicks et al. Similarly contradictory are reports on hospital admissions. While a Finish study finds no effects Lahti et al. There are very few reports on the chronic effects of DST. These are difficult to study as comparison between periods with DST and Standard Time are usually inherently confounded by seasonal effects.

The chronic effects may be small on an individual level, but they accumulate over time in individuals and both across time and space in populations resulting in big effects, the costs of which can be assessed similarly to those of insufficient sleep Hafner et al.

From a chronobiological perspective, chronic effects are very likely because, throughout the months of DST, body and social clocks are likely set to different time zones in most people, as we explained above.

Due to the fact that mostly only the transitions to and from DST in spring and autumn have been studied rather than the steady state effects in summer and winter it is still unclear whether and how much people actually adjust to DST through artificial light exposure.

When daily sleep timing as well as activity profiles are recorded for several weeks before and after the transitions, one can see that sleep times adapt relative quickly to the new social time regime — especially on workdays Kantermann et al.

An increased mismatch between body clock and social clock time during DST is supported by the only two published steady-state studies that we know of. In the first study performed in Australia, cortisol rhythms were found to be advanced by only 2 min during DST not the 1 h corresponding to full adjustment when comparing 3, summer samples taken during a 3-year-DST-test phase versus 9, summer samples during Australian non-DST summers Hadlow et al.

This finding suggests no change in body clocks despite the change in social clock during DST. In the second study, the analysis of the three different states of DST in Russia i.

The same study also found a small decrease in winter depression symptoms during perennial Standard Time Borisenkov et al. As mentioned above, any study showing long-term positive effects with the cessation of DST in autumn suggests that chronic negative effects have likely been acting throughout the months of DST. Even if the positive effects are due to sleep extension on the one night of the DST-to-Standard Time transition, they would indicate a prior sleep debt during DST Klerman and Dijk, In addition to these few studies that address chronic DST effects directly, there are four indirect ways of estimating chronic DST effects: i relative position in a time zone i.

Several studies have investigated the prevalence of different cancer types as well as general and cancer-specific mortality as a function of distance from the eastern border of the time zone: Borisenkov, ; Gu et al. All three studies conclude that risks increase and longevity decreases from the eastern to the western border of time zones.

These results suggest that the discrepancy between the social clock and the sun clock even within a time zone can have a significant effect on health; on the western edge of a time zone, social clock time is later than sun clock time as is the case during DST and at the eastern edge it is earlier.

Similar findings are reported for the incidence of winter depression, which also gradually increases within time zones, i. This is in contrast to Olders , who finds that later sunrise times, i.

The author argues that sleeping late increases REM sleep, and thus may increase depression risk and suggests that later sunrise times mean earlier rising times in relation to sunrise and therefore proposes to switch to perennial DST. It is also contrary to the well-documented, beneficial effects of early morning light in mood disorders Wirz-Justice and Benedetti, Also, in their Figure 1, which is based on 1-year averages, demonstrates only an association that does not allow causal inference.

Furthermore, there are many confounding variables that can affect the prevalence of depression such as age, sex, socioeconomic status, etc.

Later sunset times are also associated with fewer hours of sleep, poorer academic performance, and lower wages among adults in a study of people in India, Indonesia, and China Jagnani, The health effects that depend on the east-west position within a time zone described above have not yet been investigated in the CET zone. Of interest, results from the MCTQ database from the CET zone indicate that the expected negative effects may be countered by compensatory behavior.

The further west people live within the CET zone, the later they organize their lives in reference to social time. Anecdotal evidence suggests that Germans dine later than Hungarians, the French eat later than Germans and Spaniards go to dinner later than the French. CET is at the same time as 8 p. More quantifiable questionnaire results from the MCTQ support this anecdotal evidence and also show that work-start times become progressively later in Europe from its eastern to its western border Figure 3 ; Roenneberg et al.

These behavioral changes mean that people eat and work closer to their body clock time rather than the social clock time. If the negative east-west health effects shown for Russia, China and the United States turn out to be smaller in Central Europe, it could be due to this compensatory behavior and the east-west slope would be greater if the SJL was not reduced by compensatory behavior.

That human body clocks entrain to light—dark cycles as circadian clocks do in all other animals and plants is still true for industrialized societies Roenneberg et al. DST increases the discrepancy between the sun clock and the social clock and will therefore also increase the discrepancy between the body clock and the social clock , thereby also increasing SJL see above.

SJL is associated with adverse health effects: these include increased likelihood to be a smoker as well as higher caffeine and alcohol consumption Wittmann et al. There is a strong correlation between chronotype and SJL Wittmann et al. Being a late chronotype is associated with reduced health Partonen, , but it is most likely that most of the associations between chronotype and health act via SJL rather than via chronotype itself Levandovski et al.

Since DST would delay chronotypes see above , any association between late chronotypes and reduced health would be an indicator of chronic DST effects.

SJL and circadian disruption are strongly correlated with a reduction in sleep duration Foster et al. The SJL-dependent sleep loss during the workweek Figure 4A is almost compensated for on work-free days Figure 4B , so that SJL is characterized by a constant oscillation between under- and over-sleeping. Notably people suffering from less than 30 min of SJL get the longest sleep on workdays and sleep the least on their free days compared to those of other SJL categories.

Access to electrical light itself is associated with a decrease in sleep duration de la Iglesia et al. These two categories are the only ones that get on average more than 7 h of sleep on nights before workdays, all other chronotypes gets less than 7 h of sleep as SJL increases. Figure 2. A map of Europe equivalent to Figure 1 : the actual, sun-based time zones are drawn as color-coded backgrounds and the social time zones are shown in the same stronger colors in front.

Even under Standard Time, the western areas of the social time zones are far away from the respective eastern borders of the sun-based time zones A , this discrepancy increases by 1 h under DST B note that Iceland is on perennial DST.

C A solution to the problem: the political borders of Europe are actually ideal for the correct, chronobiological separations into time zones, so that in no area of Europe the social clock has to be discrepant from the sun clock by more than 30 min. Figure 3. Data were taken from the MCTQ database. For the analysis, participants had to live in the CET zone and their postal code and city name had to match as they were used to derive latitude and longitude.

The slope of the actual east-west delay gray line in work start times is 2. This delay in work-start times despite identical local times is noteworthy since the slope of sleep timing on work-free days is 3.

The geographical location of some major European cities is indicated above the graph. Data originally published in Roenneberg et al. Figure 4. DST is a seasonal time change measure where clocks are set ahead of standard time during part of the year. As DST starts, the Sun rises and sets later, on the clock, than the day before. When DST starts in the spring, our clocks are set forward by a certain amount of time, usually by one hour.

This means that one hour is skipped, and on the clock, the day of the DST transition has only 23 hours. Since DST switches usually occur at night to avoid disrupting public life, they snatch away an hour of our usual sleeping time, forcing us to adjust our body clocks. If you set your alarm to the same time as before the clock change, you will sleep an hour less. The good news is that if you work a night shift, you will get away with working one hour less that night. The DST period in the United States begins at 2 am local time, so the hour from to does not exist in the night of the switch.

It is skipped as clocks spring forward from standard time to Daylight Saving Time see table. Note: the DST period may begin and end at different local times in other countries, but the principle is the same.

In the fall autumn , the DST period usually ends, and our clocks are set back to standard time again.