Electrocardiograms of eight normal males, 21 to 28 years of age, were recorded during the wakeful state, rapid-eye-movement (REM) sleep and stage 4 sleep. The coefficient of temporal variability (CVT) was calculated twice for each individual from sixty ‘R’ to ‘R’ intervals in each of the three states. A test-retest correlation for reliability of the CVT values was also calculated. Heart-rate variability was found to be much higher in stage REM sleep than in the wakeful state or stage 4 sleep, and variability did not seem to be a measure of arousal. The lack of test-retest reliability found in the wakeful state suggests that the CVT reflects the response of the organism to uncontrolled environmental changes.