What ingredient in Benadryl makes you sleepy?

The ingredient in Benadryl that causes drowsiness is diphenhydramine, an antihistamine. It works by blocking the action of histamine, a substance in the body that promotes wakefulness and alertness. By inhibiting histamine's effects in the central nervous system, diphenhydramine induces sedation and can lead to sleepiness. This is why it is often used as a sleep aid in addition to its allergy-relief properties.