State-dependent changes in the N20 component of the median nerve somatosensory evoked potential.

Short-latency components of median nerve somatosensory evoked potentials are generally assumed to be unaffected by sleep and level of arousal. We found that sleep prolongs the latency and alters the morphology of the N20 component in normal subjects. These changes may represent differential effects of sleep on various elements contributing to generation of the N20. Failure to control for patient state may degrade the reliability of clinical somatosensory evoked potential testing.