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

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)