Adjusted mean MMSE test score (95% confidence interval) by groups of sleep duration (n = 9,237). Adjusting for age, sex, employment, occupation, education, smoking, drinking, physical activity, tea consumption, self-rated health, waist circumference, HDL- and LDL-cholesterol, fasting plasma glucose, systolic blood pressure, napping, morning tiredness, and insomnia factors by memory impairment.