The aim of this investigation was to study the circadian periodicity of ischemic stroke onset with regard to the day of the week. Six hundred and nine patients, aged from 16 to 59 years, admitted to the Helsinki University Central Hospital because of ischemic cerebral infarction were included in this study. We observed a significant heterogeneity in stroke onset between working days and holidays, including Saturdays and Sundays (p < 0.001). The onset of ischemic stroke peaked between 6 and 8 a.m. during working days and between 8 and 10 a.m. during holidays. Strokes occurred more often during the 2 h following awakening than during any other time of the day. We conclude that there is an early-morning increase in the onset of ischemic cerebral infarction among people of the working age, which varies according to the type of day probably because of differing waking times.