Differences in sun time within the same time zone affect sleep–wake and social rhythms,but not morningness preference: Findings from a Polish–German comparison study

Abstract

Morningness–eveningness describes individual preferences for activity at specified times of the day. The present research aimed to test whether sun time entrains humans and whether this effect is observable in sleep–wake timing, in the timing of social rhythm and in morningness preference. Furthermore, we tested whether different reference points (activity expressed in standard time or in sun time, morningness preference scores) provide concordant results about differences in chronotype according to longitude. University students were tested in two locations (Warsaw, Poland; Heidelberg, Germany) positioned within the same time zone but differing according to longitude, thus daylight appeared earlier in the east (Warsaw) than in the west (Heidelberg). Sampling was scheduled to obtain similar photoperiods and other environmental factors in two locations. Measures consisted of times of day when various activities occurred (e.g. going to bed, waking up, going outdoors) in the seven days prior to data collection, morningness preference, and depressiveness. Varsovians and Heidelbergers did not differ in morningness preference and depressiveness, but Varsovians, compared to Heidelbergers, undertook a number of activities (e.g. wake up, get up, having breakfast, first contact with another person and going outdoors) at an earlier clock time (21–38 min earlier, depending on activity), did not differ in starting and finishing classes/work, and were later in eating lunch and dinner. However, all the activities of Varsovians (except for going outdoors) were positioned later according to sun time reference. Thus, residents from east as compared to those from the west had similar morningness preference, were more morning positioned in many aspects according to standard time, but were later according to sun time. Results indicated university students entrained to sun time to some extent, and morning activities more coupled to sun time.