An allergy occurs in your body when foreign substances enter your immune system. The white blood cells and tissue cells release histamines, and it quickly gets into action. Thus, it causes you a range of symptoms of allergy-like running nose, sneezing, itchy eyes, chest congestion, and lots more. On the other hand, antihistamines are used to get relief from the symptoms of allergy. And it also blocks histamines from carrying out the functions in the body. Thus, the antihistamines cross the blood-brain barrier. Besides, it obstructs one of the tasks of histamines, i.e., sleepiness and wakefulness. And this disruption of histamines in the brain causes drowsiness or sleepiness.