Human daily rhythms: Clocks vs light/dark cycle

Should people living along a meridian be doing their basic activities (sleep/wake, working, eating, leaving home/coming home) at the same time? A 'yes' is an intuitive answer since solar noon happens simultaneously along the meridian. This phenomenon helps setting clock time. Therefore, a 'yes' is also pointing to clock synchronization, irrespective of latitude.

On a second thought we may understand that nobody gets to work at, say, 8am because it is four hours before noon, the bare meaning of clock ticking 8am. Instead the decision making process (whether 8am is fine, late or early) is driven by light conditions: did the Sun already rise? The answer to this question varies with latitude and season following the natural cycle of light and dark. Therefore, if this question influences human behaviour then people living along a meridian would not be doing their basic activities at the same time.

José María Martín Olalla, professor at University of Seville, addresses this issue in a paper entitled "Latitudinal trends in human primary activities: characterizing the winter day as a synchronizer" published in Scientific Reports, the Springer Nature open access megajournal. From time use surveys in 17 European countries and 2 American countries (located from 35º to 61º latitude) he characterizes laborer's primary activities and get them positioned along the daily and yearly cycle of light and dark.

Results show up latitudinal patterns tied to the light/dark cycle with the winter terminator as a source of synchronization for daily activities of laborers. Societies memorize the shortest photoperiod (daytime) of the year, the most challenging condition in one year. Winter photoperiod decreases by two hours from 40 to 54 degree latitude.

Winter sunrise (the later sunrise of the year and increasingly later with increasing latitude) triggers human activity in the morning year round and dominates morning trends. Its fingerprint can be traced on rising times, leaving home and working start times, all of them occur earlier with decreasing latitude. From 40 to 54 degree latitude, winter sunrise delays by one whole hour, the size of a standard time zone.

Winter sunset (the earliest sunset of the year, increasingly earlier with increasing latitude, it delays another whole hour from 54 to 40 degree latitude) triggers the reverse, shutdown process and dominates evening activities like stop working, coming home or dinning.

Two overturning sequences can also be identified. The first one occur at noon where lunch times exhibit both a meridional behaviour (tied to noon) and a latitudinal trend tied to the winter sunset. In this case people advance lunch times as latitude increases foreseeing the incoming dusk while people delay lunch times with decreasing latitude as light conditions do not worsen comparatively too much.

The second overturning sequence occurs at night and indoors: TV prime time marks and bedtimes are not tied to the winter sunset. Instead, they exhibit meridional behavior or trends weakly coupled to the winter sunrise. Societies are foreseeing the uprise in the following day.

The magnitude of the latitudinal gradient which dominates human activity can be comparatively traced out by observing how the terminator sweeps Europe in winter, when morning times are relatively similar as the sunrise terminator efficiently sweeps the continent, while evening times goes step by step following the sunset terminator.

Indirectly this study also inspects the role of time zone and time advance in human behaviour. The case of France, Belgium and Spain illustrates this issue. There, clocks are set one hour ahead of their physical time zone: that is an advanced clock, not an uncommon option for local time on Earth. Despite this time marks make perfect sense when properly tested against the LD cycle. That means people offset clock advancing by delaying time schedules apparently. In so doing they kept in phase with the LD cycle. This poses no harm to population. It only jeopardizes time comparisons, most notably in Spain due to its Southwestern most location. A rule of thumb valid for comparisons (both academic and non-academic) is subtracting one whole hour. That would convert "advanced clock" reading into standard time values.