To date very little research has been conducted on night-to-night variability in the incidence of insomnia. It is unclear from prior research whether subjects with primary insomnia (PI) exhibit good sleep (or better than average sleep) on some interval basis. In the present study, pilot data are provided on: (1) the frequency with which 'good sleep' occurs in subjects with PI; and (2) whether these events occur in a non-random manner. Ten subjects with PI participated in this 'naturalistic' study. All subjects completed daily sleep diaries for a minimum of 20 days. None of the subjects received treatment for their insomnia during the monitoring period. The night-to-night data were evaluated by typing each night's sleep as 'Good' or 'Bad', and then by determining the number of bad nights that occurred prior to a good night for each subject. Good and bad nights were typed in two ways: (1) using a > or =85% cut-off and (2) using a better than the individual's mean sleep efficiency (idiographic cut-off). Subjects exhibited good sleep on between 29% (>85% criteria) and 55% (idiographic criteria) of the nights evaluated. The temporal patterning analysis (based on an idiographic cut-off) revealed that better than average sleep most frequently occurred (>89% of instances) following one to three nights of poor sleep. These data suggest that insomnia severity may be mediated/moderated by sleep homeostasis and that the homeostat, or input to the homeostat, may be abnormal in patients with PI.