The prevalence of sleep apnea, sleep apneic activity, nocturnal myoclonus, and nocturnal myoclonic activity was assessed in 375 outpatient insomniacs and 150 normal controls. Only a small percentage of insomniacs (n = 8, 2.3%) and normal controls (n = 2, 1.3%) presented > or = 30 apneic events per night. Only one of these subjects, an insomniac, met the laboratory and clinical criteria of sleep apnea sufficient to recommend treatment. A limited amount of sleep apneic activity per subject (only 3-29 apneic events per night) was evenly distributed between insomniacs (n = 52, 13.9%) and normal controls (n = 22, 14.7%). Also, small percentages of insomniacs (n = 43, 11.5%) and normal controls (n = 11, 8.5%) displayed nocturnal myoclonus (five or more leg movements per hour) or nocturnal myoclonic activity (less than five movements per hour). Thus, these results do not support the claim that sleep apnea or nocturnal myoclonus are common causes of insomnia. In addition, different cutoff points of REM latency (based on first-night recordings) were associated with very low sensitivity and moderately high specificity resulting in inadequate levels of diagnostic accuracy in differentiating depressed insomniacs from non-depressed insomniacs. Finally, empirically optimized values for REM variables were less successful than similarly optimized MMPI values in differentiating depressed insomniacs from nondepressed ones. In conclusion, with our current level of knowledge, polysomnography has limited clinical usefulness in the differential diagnosis of insomnia.