Sleep studies and distribution of sleep during the day arouse high scientific interest. The aim of the study was to record the sleep habits of medical students of the University of Crete before and during the examination period. A questionnaire consisting of 14 questions and a 24 - hour daily diagram was daily filled by 493 medical students before and during the examination period. The students attended from the first to the fifth year of the medical studies, and 202 were male and 291 female. Four parameters were statistically significant (p <0.05). The mean duration of night sleep was reduced from 7.21±1.02 h before the examination to 6.14±0.63 during the examination period (p <0.001). The bed time was delayed from 1.29±01.21 a.m. before the examination to 02.28±00.58 a.m. (p <0.02). The hours of daily study were increased from 3.45±1.63 hours before the examinations to 5.98±2.02 hours during the examinations (p <0.001). Feeling sleepy during the day was increased from 36% of the days before the exams to 44% of the days during them (p <0.003). During the examination period mean duration of the students sleep was reduced, while bed time was delayed and students report tiredness at awakening in almost half of the days of this period. In conclusion examinations seem to effect the sleep habits of the students. Pneumon 2001, 14 (2): 133-138