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
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