system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do antihistamines make people sleepy?

It’s the time of year when the trees are shedding pollen to reproduce with other trees, my eyes and sinuses clearly do not consent to this airborne plant-bukkake. I’ve taken a little pink pill, and it does alleviate the allergy reaction, but it also puts me to sleep. I have stuff to do, this is annoying.
gpt
Histomines do a lot of different things for different parts of the body. In most of your body, it helps control inflammatory response (which causes allergy symptoms). 

In parts of your brain, it regulates wakefullness. 

Some antihistimes like benadryl are non-discriminatory antihistamines. This means they block the effect of histomines everywhere, including in your brain. The histomines that usually keep you awake are now blocked and that makes you drowsy.

"Less/Non-drowsy" allergy meds have less effect on those histomines while still being effective against the rest that cause allergy symptoms, making them effective but less drowsy.