Daytime sleepiness and antihistamines.

A daytime nap procedure was used to evaluate the daytime sleepiness associated with antihistamines, as well as to assess their hypnotic potential. Healthy, normal subjects received diphenhydramine (150 mg), terfenadine (120 mg), and placebo and went to bed at 900, 1100, 2000, and 2200 h with the instruction to try to fall asleep. The remained in bed for 60 min while standard sleep recordings were made. Across all conditions latency to stage 1 sleep increased significantly from nap 1 to nap 4 and the amount of sleep (all nonstage 1 sleep) decreased significantly. Over the four naps the mean latency to stage 1 sleep with diphenhydramine was significantly shorter than terfenadine and placebo, which did not differ. On the other hand, there were no differences among the drug conditions in the amount of nonstage 1 sleep. In sum, diphenhydramine at this dose produces sleepiness but shows little potential as a hypnotic, and accumulated sleep across the day makes people progressively more alert.