Can you tell me how anti-histamines make people drowsy ?

Anti-histamines: They can be h1-receptor antagonists such as Benadryl (diphenhydramine) or periactin or loratadine. Or, h2-receptor antagonists such as tagamet or ranitidine. Or, h3/h4 receptor antagonists (experimental) such as thioperamide. Or, atypical antihistamines such as catechin. Or, others such as cromolyn.

Yes: Histamine's action on the brain's histamine receptors keeps you alert. When the antihistamine enters the brain , it counteracts this stimulatory effect thereby making you drowsy. Since the 2nd generation antihistamines do not enter the brain , they are not expected to cause drowsiness.