Poor sleep hygiene is related to excess body fat in young women. Physical activity is an important part of energy balance. The purpose of this study was to evaluate the relationship between sleep and next day physical activity under free-living conditions in young women.


Three hundred and thirty young adult women (17-25 years) participated in this cross-sectional study. Both sleep and physical activity were measured objectively for seven consecutive days by accelerometry.A structural equation model (SEM) was used to model the relationship between the driving factor of sleep (either total sleep or morning wake time) and the amount of non-sleep sedentary (SED) and moderate-to-vigorous (MVPA) activity each day.


The average sleep time per night was 7.1 ± 0.7 hrs, the average bedtime was 12:42 AM ± 1:37 hrs and the average wake time was 8:43 AM ± 0:59 hrs.In the SEM model with sleep duration as the driving factor, βSED and βMVPA were -0.415 and -0.093 respectively (P ≤ 0.05). For every hour a participant slept there was 60 × 0.415 = 24.9-minute reduction in sedentary time and a 60 × 0.093 = 5.58-minute reduction in MVPA. The estimates of βSED and βMVPA when wake time was the driving factor was -0.636 and -0.149, respectively. This indicates that a wake time that is 1 hour later is associated with a 60 × 0.636 = 38.2-minute decrease in SED and a 60 × 0.149 = 8.9-minute decrease in MVPA (P ≤ 0.05).


Previous research has shown that sleep is related to body weight and fat in young adult women. However, based on the results of this study it seems that this relationship is not a result of reduced physical activity. Young women who sleep longer or wake up later tend to have less MVPA.