There is a hypothesis which proposes that the duration of daylight following birth sets the internal clock and this is the basis of the effect of season-of-birth on chronotype in adulthood.
An imprinting-like phenomenon may occur during a “sensitive phase” during the 3 months after birth, a significant period of time during which an individual’s sleep–wake cycle is being developed, and the photoperiods in the weeks following birth have an influence on chronotype.
Individuals born during long or lengthening photoperiods (summer or spring) would set their internal clock with longer days in comparison to those born during the shorter or shortening photoperiods characteristic of winter or autumn.
However, it is possible that grouping individuals by season of birth may be masking the effect of PAB (photoperiod-at-birth) on chronotype as photoperiod varies even within a season.
Researches show that in the Northern hemisphere, morningness was linked with winter and autumn births (September to March), while eveningness was more frequent in those born in spring and summer (March to September). In the Southern hemisphere, the pattern was similar as those born from March to September (winter and autumn) were more frequently morning types, whereas those born from September to March (spring and summer) were linked with eveningness.
They suggested that time of sunrise, sunset and daylength at birth had a significant impact on chronotype, factors all determined by both latitude and longitude which have also been shown to affect chronotype.