The study assessed the effect of physical fitness and body composition on sleep and the nighttime secretion of the hormones, human growth hormone (hGH), prolactin, and cortisol. Two groups of 17 subjects, one of fit athletes and the other of unfit nonathletes, were selected so that the groups were matched for weight, height, lean body mass (LBM), and fat levels. Subjects slept in a sleep laboratory for 3 nonconsecutive nights: 1 adaptation night and 2 experimental nights. On 1 experimental night blood samples were collected; on the other, baseline sleep was assessed and the catheter was not inserted. Weight and height were measured and LBM assessed by 24 h urinary creatinine. The effect of physical fitness was tested by a comparison of the two groups; body composition was assessed by correlation analyses. Physical fitness did not have a significant effect on either sleep or hormone levels, although in the latter case the results were marginal. In contrast, body composition was related to both sleep and hGH. Percentage LBM was negatively correlated with slow-wave sleep and positively correlated with hGH levels. These results were significant for all subjects combined and for the fit group, although not the unfit group alone.