(iii) The body clock determines the organism’s internal time as defined by the circadian clock. (ii) The social clock shows the local time determined by policy in form of devices on walls, on wrists or in phones it is a social construct referring to the sun time at the meridian that was chosen for that time zone (see below). (i) The sun clock shows the local time of the apparent progression of the sun noon being when the sun is highest and midnight being exactly half way between dusk and dawn. We will repeatedly refer to three different clocks or time frames here. The opposite side of Earth from Greenwich forms the date line and conveniently consists almost exclusively of Pacific Ocean water. Prague for example, is one sun-hour to the east of Greenwich, St. Meridians are imaginary lines that run between the North and the South Pole and cut the earth into 360 even “apple slices.” Theoretically, time zones (for the social local clock time) are centered around every 15th meridian, stretching half a sun hour (7.5 longitudinal degrees) to the east and half a sun hour to the west. Since 1884, the world’s reference clock ticks on Observatory Hill in Greenwich east of London – it defines the zero meridian. The current 24-h-day translates to an angular velocity of 4 min per longitudinal degree, so the Earth rotates by 15° every hour. When the first biological clocks developed to organize physiology on a daily level (i.e., circadian clocks), probably something like 3 billion years ago in ancestors of today’s cyanobacteria ( Dvornyk et al., 2003), days on Earth were 22 h or even shorter ( Williams, 2000) days have lengthened by approximately 2 ms every century since. Notably, Earth’s day was not always 24 h. Our Earth takes (at present) 24 h for one rotation. After decades of measurements, this action was taken at a conference in Washington DC in 1884 to facilitate communication and travel between places with different sun times. The issue of Daylight Saving Time (DST) is an indirect consequence of dividing the surface of Earth into time zones.

We address numerous fallacies that are propagated by lay people, politicians, and scientists, and we make suggestions of how problems associated with DST and time-zones can be solved based on circadian biology. Here, we give the necessary background to understand how the circadian clock, the social clock, the sun clock, time zones, and DST interact. Although DST has always been a political issue, we need to discuss the biology associated with these decisions because the circadian clock plays a crucial role in how the outcome of these discussions potentially impacts our health and performance. A wave of DST discussions seems to periodically sweep across the world. Related to the discussion about DST is the discussion to which time zone a country, state or region should belong: the state of Massachusetts in the United States is considering switching to Atlantic Standard Time, i.e., moving the timing of its social clock (local time) 1 h further east (which is equivalent to perennial DST), and Spain is considering leaving the Central European Time to join Greenwich Mean Time (GMT), i.e., moving its social timing 1 h further west. Some, like Japan, that have not used DST over the past decades are thinking about introducing this twice-a-year change in clock time, while others want to abolish the switch between DST and Standard Time, but don’t agree which to use: California has proposed keeping perennial DST (i.e., all year round), and the EU debates between perennial Standard Time and perennial DST. Many regions and countries are reconsidering their use of Daylight Saving Time (DST) but their approaches differ.

Sleep time zone changes