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Seasonal clock changing helps to synchronize human sleep/wake cycle across latitude

May 20, 2019

Scientific Reports (Nature Research) released this month a paper by prof. José María Martín-Olalla (Universidad de Sevilla) where seasonal similarities between the sleep/wake cycle in Subtropical, pre-industrial (without access to electricity) and Extratropical, industrial societies are analyzed.

In winter, results show that the sleep/wake cycle is dominated by sunrise. Wake-up times tend to occur during the winter twilight regulated by the circadian photorecpetive mechanism. Bedtimes tend to occur eight hours earlier or sixteen hours later, in the middle of the winter night, regulated by the homeostatic mechanism that make us feel tired after a prolonged wake.

This setting delays the sleep/wake cycle as latitude increases following the delay in the winter sunrise time.

From the Subtropic to fifty degrees latitude (the latitud of London or Berlin) the lag amounts to two hours.

This winter lag is less adecuate in summer due to the change in two natural magnitudes. Sunrise time now advances with increasing latitude. Noon insolation increases significantly in Extratropical region. Below the 47th parallel (which divides Switzerland) noon insolation in summer is greater than at the Equator.

Although Extratropical societies have been using different seasonal mechanisms to mitigate in summer the winter lag, contemporary societies achieve that by means of Summer Time Arrangements or Daylight Saving Time (DST). The spring clock-change advances the phase of the sleep/wake cycle so that it shows a great coincidence from the Equator to 55 degrees latitude. The sleep/wake cycle is dominated by distance to noon. Wake-up times occur some six hours before noon, bedtimes occur some ten hours after noon. That means 7am and 11pm local time if DST applies.

For the analysis prof. Martín-Olalla retrieves data from seven previous reports involving nine pre-industrial societies. The Tsimané (in Bolivia), the Hadza (in Tanzania), the San (in Nambia), the Toba/Qom (in Argentina) and the Quilombolas (in Brazil), also the locations of Fondwa in Haiti, Milange and Tengua in Mozambique, Mandena in Madagascar and Chico Mendes in Brazil.

Almost simulataneously the European Journal of Internal Medicine (Elsevier) published a Comment Letter where prof. Martín-Olalla explains how latitude influences the different ways by which contemporary societies adapt to the seasonal cycle, including their preference for continuing or discontinuing seasonal clock-chaning. Prof. Martín-Olalla ends up suggesting that European Commision must allow opt-outs in this issue and let every Member State to decide on this issue. The seasonal cycle of light and dark at the lowest bound of latitude in the Union (the Iberian peninsula) is fairly different to that observed in the highest bound (Scandinavian peninsula) in terms of seasonal change of sunrise time and of the efficiency of the insolation.
The impact of latitude in human primary activities was already analyzed by prof Martín-Olalla in a previous paper published in 2018 also in Scientific Reports.

University of Seville

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