Our inner biological clock seems to be determined genetically.

Now, a study investigating the activity patterns of thousands of people from different geographical locations discovered that the human circadian clock tunes to the local sun, even if the people live and work using a common "social time" determined by time zones.

The research also pointed that urban population experiences a lower influence of local sun time than those living in villages or small towns.

A natural discrepancy is inherent between the natural-light linked to local sun time and the "social" driven clock time as our time is scheduled according to time zones, the same over multiple longitudes, rather than to local sun time, which gradually changes over longitudes.

This discrepancy varies with the year's period and one's location within a time zone, but can be quite big: in some locations, midnight determined by time zone can be well beyond an hour away from "mid-dark."

In order to see how these two cues (the natural and the social) interact on our circadian rhythms, the investigators compared the circadian behaviors of people experiencing these influences in different parts of the same time zone.

The research team compared chronotypes, questionnaires that monitored times of habitual activity and rest during work days and free time, of over 21,000 persons from different geographical locations in Germany.

Two groups were formed: those living in areas with a population of 300,000 or less and those from larger cities.

The chronotypes of the first group proved to be tightly coupled to sun time, but in the second group, they showed a weaker link to sun time.

Urban population was typically less exposed to sun light than individuals from less populated areas.

Previous research pointed that when the sun light influence on circadian clock turns weaker, the circadian rhythm is shifted later in the day.

This analysis enhanced it: the more urban the population, the later the clock.