Theories of sleep typically view sleep as having a compensatory relationship to wakefulness. This hypothesis leads to the prediction that physical exercise will cause both sleep duration and the amount of slow wave sleep (SWS) to be higher on the following night. Three previous reviews of the literature, while not agreeing in detail, have generally supported the hypothesis. It is argued that, in two of these reviews in particular, the extent of the positive evidence was overstated. Consistent with this interpretation the subsequent literature is also negative with regard to the hypothesis. Further, physical fitness, which occurs as a result of long periods of physical training, is not consistently associated with either longer sleep durations, or higher levels of SWS. While there is some evidence that exercise may have indirect effects on sleep under a limited range of circumstances, there is no evidence to support the hypothesis that physical exercise directly causes, or is reliably associated with, longer sleep durations or higher levels of SWS.