I'm a frequent black coffee enjoyer. For the longest time, I'd normally average 1 to 2 cups in a day of the stuff.
This year, I've been having more issues than usual keeping focused on tasks, which I'm pretty sure is because of ADHD reasons (was diagnosed as a kid when it was called ADD, and has been an issue for basically my life). I drink coffee because I like the flavor, but it does absolutely help for the span of, say, 3 to 4 hours of focusing before it wears off.
One and a half weeks ago, I started an experiment, basically cutting off all coffee intake aside from decaf (had 3 cups of mild caffeinated tea for some dire days of focus needed) because I felt it was likely affecting my ability to focus and stay awake on things I needed or wanted to get done during the day, plus the caffeine crashes render me a puddle of a human being staring off into the endless void.
First few days off of coffee, I certainly felt much more sleepy in the middle of the day. Lost some productivity, but over time I felt a bit more... level headed? Basically weening off of the caffeine meant the energy i had was more evenly spread through the day. It didn't solve my inability to focus, but at least it was consistent throughout the day.
This past Monday since starting the cutoff, I had my first cup: I felt fine! In fact, I felt focused for that bit of time! Nice!
Tuesday: Pretty tired, but had another casual cup. Kinda woke up more again, but whew I couldn't focus as much.
Today, Wednesday: ... brain was might as well have been completely shut off. Spacing out haaard. Nothing productive got done and I wanted to crawl back in bed all day...
I made myself decaf just now. I think I'm gonna go a bit longer with no regular coffee and see if things change again.