A side effect of Benadryl is that it causes drowsiness, which can make people fall asleep. Numerous over-the-counter sleep aids contain antihistamines such as diphenhydramine and doxylamine succinate because of their potent sedative effects.
Histamines are chemical compounds produced by the body and are activated when a potentially dangerous foreign substance is detected. When someone has a cold or allergy, histamines are responsible for inflammatory responses such as sneezing or runny noses. Antihistamines such as Benadryl ease the symptoms by blocking histamine receptors. Antihistamines cannot discriminate between receptors, so they also block receptors in the brain that regulate sleep and wakefulness. Some newer antihistamines cause less drowsiness.