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Table 1 Characteristics of subjects included in the present study at each of the four measurement points. Persons with missing data on sleep length are excluded

From: Changes in self-reported sleep duration with age - a 36-year longitudinal study of Finnish adults

Year of questionnaire Number of individuals Women (%) Age (mean ± 1.96 x Standard deviation) in years
Range of age (birth years)
1975 30,915 51.4 35.7 ± 14.6
18–95 (1880–1957)
1981 24,535 51.7 40.5 ± 13.7
24–101 (1880–1957)
1990 12,450 54.4 43.9 ± 7.8
33–60 (1930–1957)
2011 8334 55.4 59.7 ± 3.7
54–66 (1945–1957)