I personally wouldn't do that with Benadryl. Everyone is different - but the side effects ime/o can be troublesome. You may want to research BL for other reports of doing this. Just type 'Diphenhydramine' in search area, upper right side of page.
^ what he said (and everyone else for that matter.) The few times I have tried high doses of benedryl around ~200mg it was waves of dysphoria, then mild euphoria and back to dysphoria. I also get some extreme body-anxiety from it, and sleep became impossible for like 5 hours. Once it kicked in, felt like I hadn't slept for 3 days, that "eyes staring off into nowhere" feeling I guess you could say.
I didn't see ghosts or monsters or anything though and no shadow people; it was just a very heavy body high. I shant be trying that again. Just be safe buddy don't go driving or anything like that and as the other member said don't go mixing it with alcohol. If you can't sleep or feel weird, turn off the lights, lay down and put some headphones on with your favorite music, that always helped me at least.