Fig. 2. Remifentanil effect on sleep. Polysomnography from the nights with a saline infusion versus the nights with a remifentanil infusion demonstrates a significant decrease in the percentage (mean ± SD) of time spent in REM sleep. * P < 0.05. REM = rapid eye movement sleep; Remi = remifentanil–placebo; Saline = saline–placebo; SWS = slow wave sleep.