Abstract Introduction Short sleep duration has been linked to obesity risk in adolescence. However, most research has focused on potential changes in appetite/intake, rather than physical activity or sedentary behaviors. It remains unknown if, in the daily lives of adolescents, sleep restriction increases moderate- to-vigorous physical activity (e.g., by providing more time for it) or discourages such activity (in favor of sedentary behaviors). This was the first study to use gold-standard objective measures to assess cause-and-effect relationships between sleep duration and the resulting activity levels of adolescents in the naturalistic environment. Methods N=104 healthy teens (ages 14–18) completed the 3-week within-subjects crossover sleep manipulation experiment during the summer. Following a 7-night a sleep stabilization week, teens were randomly assigned to 5 nights in Short Sleep (6.5hrs sleep opportunity) or Healthy Sleep (9.5hrs sleep opportunity). Following a 2-night “washout” period, they crossed over to the alternate sleep condition. Throughout the study, they wore validated waist-worn accelerometers to objectively measure sedentary and physical activity levels, and wrist-worn actigraphs to confirm adherence to their sleep condition. Results When in Short Sleep (vs. Healthy Sleep), teens on average slept 112 minutes less (p<.0001, d=1.72) per wrist actigraphy. Waist-worn accelerometers reflected 99 more minutes in sedentary behavior (p<.0001, d=.97), and 16 more minutes in light physical activity (p=.002, d=.31) during short sleep. Teens did not differ in moderate-to-vigorous physical activity between conditions (p=.95, d=.03). Conclusion Among healthy adolescents, a realistic dose of sleep restriction did not affect moderate-to-vigorous physical activity levels, but did sharply increase time in sedentary behavior. Given the negative weight and health consequences of sedentary behavior, these results have practical implications for obesity prevention/intervention efforts. They suggest that extending teen sleep may neither encourage nor discourage healthy physical activity, but may help curb unhealthy behaviors (e.g., sedentary behavior). Support R01 HL120879
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