2. Study Variables

We assessed sleep duration by asking "On average, how many hours a day do you usually sleep?" through a self-administered questionnaire. Based on their responses, we categorized study subjects into three groups according to their sleep duration: short (<7 h/d), adequate (7–8 h/d), and long (>8 h/d).

Venous blood was drawn after at least eight hours of fasting, and glucose levels were measured using a Hitachi Automatic Analyzer 7600 (Hitachi, Tokyo, Japan) in a qualified central laboratory. IFG was defined as fasting glucose levels between 100–125 mg/dL, according to criteria from the American Diabetes Association.4)

Level of education, monthly family income, marital status, ever-smoking, current alcohol use, physical activity, stress recognition, and menopausal status were assessed using a self-administered questionnaire. Education level was categorized into two groups (<high school, or ≥high school). Monthly family income was categorized into four groups (≤1,000,000, 1,000,001–2,000,000, 2,000,001–3,000,000, or ≥3,000,001 Korean won). Marital status was categorized as ever-married or never-married. Ever-smoking was defined as having smoked at least 100 cigarettes in a lifetime. Current alcohol use was defined as at least one drink per month for the previous year. Regular physical activity was defined as participating in moderate activity for more than 30 min/d at least five days per week, or vigorous activity for more than 20 min/d at least three days per week. Stress recognition was assessed by asking "How much stress do you usually experience in your daily life?", and then categorized into two groups (high or low levels of stress). Menopausal status was categorized as premenopausal or postmenopausal states using a self-administered questionnaire. Body mass index (BMI, kg/m²) was calculated using measured weight and height. BMI was categorized as underweight (<18.5 kg/m²), normal weight (18.5–22.9 kg/m²), or overweight/obese (≥23 kg/m²), according to recommendations from the Korean Society for the Study of Obesity.17)