Modeling the cardiometabolic benefits of sleep in older women: Exploring the 24-hour day.

STUDY OBJECTIVES Activities throughout the day, including sleep, sedentary behavior (SB), light physical activity (LIPA), and moderate to vigorous physical activity (MVPA) are independently associated with cardiometabolic health. Few studies have examined interrelationships between sleep and 24-hour activity and associations with cardiometabolic risk. The objective of this study is to understand how replacing time in SB, LIPA, or MVPA with sleep impacts cardiometabolic risk. METHODS Women's Health Initiative OPACH Study participants (N=3329; mean age=78.5±6) wore ActiGraph GT3X+ accelerometers 24 hours/7 days. Adjusted linear regression estimated the relationship between sleep duration and cardiometabolic markers. Separately for shorter (<8 hours) and longer (≥8 hours) sleepers, isotemporal substitution models estimated the cross-sectional associations with cardiometabolic markers with reallocating time in daytime activities to or from sleep. RESULTS Longer sleep duration was associated with higher insulin, HOMA-IR, glucose, total cholesterol, and triglycerides (all p <0.05). The associations between sleep duration and C-reactive protein, waist circumference, and BMI were U-shaped (both p <0.05). For shorter sleepers, reallocating 33 minutes of MVPA to sleep was associated with higher values of insulin, HOMA-IR, glucose, triglycerides, waist circumference, and BMI (0.7%-11.5%). Replacing 91 minutes of SB time with sleep was associated with lower waist circumference and BMI (-1.3%, -1.8%). For long sleepers, shifting 91 minutes of sleep to SB was associated with higher waist circumference and BMI (1.3%, 1.4%). CONCLUSIONS This is one of the first isotemporal analyses to include objectively measured sleep duration. Results illuminate possible cardiometabolic risks and benefits of reallocating time to or from sleep.