In nonrapid eye movement (NREM) sleep, electroencephalographic cyclic alternating patterns (CAPS) express the organized complexity of arousal-related phasic events. As a translation of sustained instability of the arousal level, CAPS increase under perturbation and decrease under sleep-promoting conditions. After adaptation to the sleep lab, 18 subjects (12 with persistent psychophysiological insomnia and 6 without sleep complaints), all aged 40-60 years, underwent a random sequence of two nonconsecutive nocturnal recordings, one under placebo and one under an imidazopyridine hypnotic agent (zolpidem). The choice of this drug was oriented by its capability to warrant, at the selected dose of 10 mg, a physiological profile of sleep. Polysomnographic parameters and self-report of sleep quality by means of visual analogue scale (VAS) were assessed by analysis of variance followed by a Scheffé test when F statistics were significant. The placebo nights of controls showed a significantly longer duration of total sleep time (+60 minutes) compared with the placebo nights of insomniacs. However, no relevant differences emerged from the other traditional polysomnographic variables. Conversely, CAP rate (CAP time x 100/NREM sleep time) showed highly sensitive and consistent modifications. Under placebo, CAP rate was significantly higher among insomniacs compared with controls (62.2% vs. 38.5%). Under medication, CAP rate dropped to 26.8% in insomniacs, whereas it was only moderately reduced in controls (31%). CAP rate values paralleled the self-rating estimation of sleep quality, but the VAS means differed significantly only among insomniacs where a sharp average fall from 45 mm (placebo nights) to 15 mm (zolpidem nights) could be detected. Our findings suggest that CAP rate in sleep analysis can represent a useful tool for the diagnosis of insomnia and for gaining insight into the therapeutic efficacy of hypnotic-sedative drugs.