Why do some antihistamines make me drowsy?

Cross into brain. Older antihistamines, such as diphenhydramine, or chlorpheniramine, actually cross the barrier between the bloodstream and the brain. This causes many people to feel drowsy or sedated. It can make some people irritable, or even cause insomnia. For this reason, the "non-sedating" antihistamines were developed. These medications cause much less drowsiness, and are better tolerated.
Antihistamines. Because they cross the blood brain barrier and trigger receptors that can lead to sedation. The only one that does not cross the blood brain barrier is allegra/fexofenadine.