As a result of sistematic research during the last decade it has been found that a great majority of the population is effected by seasonal changes at different degrees. The degree at which seasonal changes effect mood, energy, sleep length, appetite, food preference or social activity is called seasonality. The present study aimed to investigate the seasonality pattern of university students in Istanbul. 228 students of Marmara University Faculty of Medicine were invited to complete the Turkish version of the Seasonal Pattern Assesment Questionnaire (SPAQ) of Rosenthal et al. % 39.5 of the students reported that they perceived the seasonal changes as a problem at different degrees (15.8 % mild, 14.0 % moderate, 8.3 % marked, 1-3 % severe or disabling). Mean global seasonality score was 11.6 and a large percentage changes in their sleep length (60.5 %), social activity (82.0 %), mood (71.1 %), body weight (45.2 %), appetite (57.0 %) and energy level (76.8 %). The months in which the subjects felt themselves the best and where they were most social were found to be spring and summer months, whereas autumn and winter months were the ones in which they felt themselves the worst. These results showed that a large percentage of the students experience seasonality in their mood and behavior.