Short-acting benzodiazepine hypnotics may phase-shift circadian rhythms and improve adaptation of sleep patterns to abrupt time shifts, depending on the timing of administration. The aim of the present study was to determine whether bedtime administration of zolpidem, a non-benzodiazepine hypnotic, causes alterations in circadian rhythmicity or in the normal interactions between sleep and hormones. Eight normal women (aged 21-33 years) each participated in a baseline study and a study with zolpidem administration. On each occasion, blood samples were obtained at 20-minute intervals for 25 hours, starting at 1000 hours. Zolpidem (10 mg) was given orally at 2245 hours. Zolpidem administration was associated with an increase in stages III + IV sleep. Cortisol, melatonin, thyrotropin and growth hormone profiles were similar in both experimental conditions. In contrast, though remaining in the normal range, the nocturnal elevation of prolactin was enhanced two-fold in all subjects after zolpidem during early sleep, and prolactin levels were still 50% higher than baseline in late sleep. Morning levels were similar in both studies. In conclusion, bedtime administration of 10 mg zolpidem, a standard clinical dosage, systematically induces a transient moderate hyperprolactinemia, but does not alter other sleep-related hormonal secretions or endocrine markers of circadian rhythmicity.