BACKGROUND:Sleep quality in people with cystic fibrosis (CF) is known to be poor, whereas participating in regular physical activity is associated with less decline in lung function (forced expiratory volume in 1 sec [FEV1]). The relationship between sleep quality and physical activity in people with CF is unknown. METHODS:Secondary analysis of sleep and activity data collected via actigraphy. Adults with CF in stable health, participating in a study of physical activity (including assessment of exercise capacity), completed 7 d of activity and sleep assessment (SenseWear Armband [SWA]; BodyMedia). Sleep characteristics were derived from accelerometer positional data and registration of sleep state by the SWA, determined by energy expenditure. RESULTS:Sleep and activity data were available for 47 participants [n = 28 male; mean ± standard deviation age = 29 ± 8 yr; median (IQR) FEV1 = 60 (50, 82) % predicted]. More fragmented sleep was associated with poorer exercise capacity (rs = -0.303, P = .04), less time spent in moderate-vigorous physical activity (rs = -0.337, P = .020), and poorer FEV1 (rs = -0.344, P = .018). Regression analysis showed that less fragmented sleep was an independent predictor of more total daily activity time (β = -1.0, standard error [SE] of β = .4, P = .02) and trended toward significance for more moderate-vigorous physical activity (β = -.3, SE of β = -.26, P = .08). Greater total sleep time and sleep efficiency were related to better exercise capacity and lung function (P < .05). CONCLUSION:This secondary analysis demonstrated a modest relationship between sleep parameters and physical activity and exercise capacity in adults with CF. Future studies of interventions to promote physical activity participation in this group should consider the relationship between sleep and activity performance.