There have been relatively few robust studies of the effects of CPAP therapy on vigilance and daytime function. This article will use all known published and unpublished randomized clinical trials on this area. There is good evidence that CPAP helps subjective sleepiness, objective sleepiness, quality of life, mood and driving performance. However, CPAP does not appear to normalize objective sleepiness. Many of the improvements are directly related to the nightly use of CPAP. Recent data suggest that increasing CPAP usage improves outcomes.