Reallocating sedentary time to sleep or physically active behaviors: associations with body mass index in college students

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More than 160 million US adults aged 20 years and older are overweight or obese. The greatest increase in the prevalence of overweight and obesity has occurred among young adults aged 18 to 29 years. College students represent a subpopulation at a higher risk for excess weight gain, which is often perpetuated by daily health behaviors, particularly, how time is spent in sleep, sedentary time (SED), and physically active behaviors. This study applied a novel isotemporal substitution model approach to investigate how reallocating time spent in SED activities to sleep and physically active behaviors influenced body mass index (BMI). College-age (20.1±1.5 years) students (n=1,533) of normal weight ( (BMI=24.4±4.7 kg/m2) provided self-reported BMI (height and weight), sleep, SED, and physical activity data anonymously through an online survey. Sleep and physical activity (SED and physically active behaviors) were assessed via the Pittsburgh Sleep Quality Index and International Physical Activity questionnaires. Sleep (r=-.070) and moderate-to-vigorous intensity physical activity (MVPA) (r=-.068) behaviors were weakly but significantly associated with BMI (all P<.05). SED (r=.043) and light-intensity-physical activity (LPA) (r=-.014) behaviors were not associated with BMI (all P>.05). In both the single and partition models, sleep (B=-.223 and B=-.238) and MVPA (B= -.333 and B=-.348) were inversely associated with BMI (all P<.05). Among the total sample (BMI: 24.4±4.7 kg/m2), reallocating 60-min of SED behavior with sleep (B=-.277, 95% CIs: -.461, -.093) or MVPA (B=-.386, 95% CIs: -.635, -.147) resulted in small but significant reductions in BMI. When limited to individuals with overweight and obesity (n=543, BMI: 29.2±4.3 kg/m2), reallocating 60-min of SED behavior with sleep (B=-.384, 95% CIs: -.667, -.108) or MVPA (B = -.796, 95% CIs: -1.15, -.436) resulted in small to large reductions in BMI, with the greatest effect coming from MVPA in overweight/obese individuals, was inversely associated with a lower BMI. Reallocating 60-min of sedentary time with sleep or MVPA produced favorable effects on BMI among college students. Reductions in BMI were greater among overweight and obese individuals, especially when SED was replaced with 60-min of MVPA.

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