First generation H₁ histamine receptor antagonists, such as d-chlorpheniramine (d-CPA) and diphenhydramine, produce drowsiness in humans. They are currently used as over-the-counter sleep aids. However, the mechanisms underlying drowsiness induced by these H₁ histamine receptor antagonists remain obscure because they produce heterogeneous receptor-independent actions. Ketotifen is a second generation H₁ histamine receptor antagonist which is more permeable to the brain than newer H₁ histamine receptor antagonists. Therefore, to access sleep-inducing profiles by H₁ histamine receptor blocking actions, the present study compared the dose-dependent effects of diphenhydramine and ketotifen (1-40 mg/kg, intraperitoneal injection at dark onset time) on daily sleep-wake patterns in rats. Ketotifen dose-dependently decreased rapid-eye-movement (REM) sleep and increased non-REM sleep by amplifying slow-wave electroencephalogram powers. Diphenhydramine at 4 mg/kg transiently increased non-REM sleep and reduced REM sleep similar to the effects of ketotifen. The larger injections of diphenhydramine (10-40 mg/kg), however, reduced non-REM sleep, abolished slow-wave enhancements and facilitated wakefulness. The bi-directional action of diphenhydramine on sleep is similar to our former results using d-CPA. Taken together, the arousal effects caused by over-dose administrations of the first generation H₁ histamine receptor antagonists may be mediated by H₁ histamine receptor-independent actions. To further examine the tolerance of ketotifen-induced sleep, 3 mg/kg ketotifen was injected daily for 5 days 3 h before light onset time. These experiments consistently enhanced non-REM-sleep at the end of the active phase of rats, suggesting that ketotifen may function as a desirable sleep aid although the coincidental REM sleep reduction requires attention.