It is widely accepted that the conventional polysomnographic parameters (macrostructure of sleep) supply only rough information for clinical purposes. In particular, they often appear inadequate to support a diagnosis of insomnia or the effectiveness of a hypnotic compound. In the past years, attention has been focused on the microstructure of sleep, and especially on the periodic distribution of arousal-related phasic events known as Cyclic Alternating Pattern (CAP). This microstructural rhythm is not only a physiological component of normal NREM sleep, but it also appears highly sensitive in the detection of disturbing factors and drug manipulation. Regardless of the specific context, CAP always translates a condition of arousal instability during sleep. Accordingly, the higher the amount of CAP, the poorer the subjective quality of sleep. In young adults, the physiological amount of CAP Rate (percentage ratio of CAP time to NREM sleep time) ranges around 25\%, while CAP Rate rises to 55\% when sleep is perturbed by continuous white noise (situational insomnia). The analysis of CAP Rate within this framework of situational insomnia is recommended for evaluating the effects of hypnotic drugs under controlled experimental conditions. Therapeutical doses of zolpidem preserve the regular course of sleep both at the macro- and at the microstructural level, when sleep is recorded under basal conditions. In contrast, during acoustic perturbation, zolpidem reduces the pathological amounts of arousal instability by lowering the values of CAP Rate to 38\%.(ABSTRACT TRUNCATED AT 250 WORDS)