Sleep deprivation in the rat: an update of the 1989 paper.

The introduction to this series of papers20 emphasized that sleep deprivation studies are essentially correlational. We administer stimuli that produce the responses of sleep reduction and the responses of physiological or behavioral change. These are correlated responses. We cannot confidently interpret the physiological and behavioral changes as effects of the sleep loss until we can discount the plausibility of other putative mediators of the physiological and behavioral change that are produced by the experimental situation. Two alternative mediators have been suggested: loss of circadian rhythm and stress.