system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do so many medicines (pain pills, muscle relaxers, antihistamines,etc.) have a side effect of sleepiness?
gpt
All of the types of medicines you described work by basically stopping your body from doing something it would normally be doing, often by interfering with the ability of your nervous system to communicate the messages necessary to do it.  A side effect of that can be to generally slow down your nervous system overall, which manifests (among other ways) in you feeling sleepy.