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One dimension of human behaviour is characterized by seasonal changes, another by diurnal preference on a timeline from morning to evening hours. The degree of seasonality (n=20,792), as well as the diurnal preference (n=17,386), among adults living in Finland were analysed using the data derived from five population-based health examination surveys conducted during the years of 2000 to 2017. On average, the seasonal variation in mood and behaviour was experienced as a problem by 25% of the study participants, whereas 9% had no variation, 7% reported seasonal variation in their mood and behaviour to the extent that equalled the severity of subsyndromal seasonal affective disorder (SAD) and 2% that of SAD. On average, 15% of the study participants assessed themselves to be a definite evening type of person (night owls). Of the night owls, 9.7%, 3.2% of the intermediate chronotypes and 1.6% of the morning larks reported having seasonality which equalled the severity of SAD. These two behavioural dimensions can be screened easily with short self-report questionnaires, the use of which might benefit not only health promotion but also disease prevention. There is a reason to encourage their routine use in healthcare services.