Season of birth modulates morningness-eveningness preference in humans

The aim of present work is to test if circadian typology couId be modulated by environmental light-dark cycle acting at the time of birth. A sample of 1584 university students was administered the Morningness-Eveningness Ques tionnaire. Subjects were subdivided in four season groups, according to the birth date. We found more morning types among the students born in autumn and winter than in spring and summer, while we found an opposite pattern fa r evening types. The exposition to a decreasing photoperiod at birth seems to favour a phase advance of the biological clock (morning typology), while an increasing photoperiod seems to favour a phase delay (evening typology) of the human circadian system. Nevertheless we found no differences if fem ale subjects separately were considered, as though the circadian typology o f human females was not modulated by seasonal photoperiod at birth. (C) 199 9 Elsevier Science Ireland Ltd. All rights reserved.