I went minimal caffeine but not zero, so I still had some decaf tea and coffee, and also small amounts of chocolate, so I estimate my daily intake was probably in the <10mg range, as opposed to my normal daily intake of roughly 50-100mg (1-2 bags worth of black tea).
Some things I don't have answers on. After about two days on no caffeine I had a mild headache. Could have been related, or could have just a random mild headache that happens occasionally after a day of staring at a screen. Several other days that week I had a hard time feeling motivation to get things done at work as well as I like to. Could have been related, or could be that I had a couple daunting projects that I was procrastinating, which definitely affects my brain for other things.
Some things I did answer. I was able to prove my claim that I've made for years that although I require tea in the morning to feel properly ready for the day, it's not the caffeine that I rely on, it's the ritual of sitting down and relaxing and sipping a warm hot drink with the right taste and feel. I spent a couple days not feeling quite right, because caffeinated black tea tastes different than decaffeinated black tea, before I explored my tea cupboard and found a decaf breakfast that is close enough, and thoroughly enjoyed that for the rest of the week. Unfortunately, it's a finely ground loose tea and so I didn't have it at work. I have some bags of a well-reviewed brand on order to test for future, though.
I observed that while I was slightly less productive (unsure if related), I was also a little less anxious overall, which is interesting because I've generally considered myself to have high functioning low anxiety all the time. So for times that I'm feeling especially stressed, going decaf again might be a useful thing for me.
When I added it back in, I was able to observe the slight elevation in alertness and concentration in a few hours following, which I appreciate and intend to keep using. I also recently accidentally gave myself a reminder to stay strict on my no caffeine after noon rule, as on a day recently when I consumed a bunch of coffee ice cream in mid-afternoon, I then completely failed to sleep well that night.
In some ways alcohol is pretty easy for me to go off of for short periods of time, because I tend to drink primarily when I am socializing and after i'm done Doing Things for the day. In a week I think only had to turn down one usual outing to grab a beer.
I think I may have slept better eventually and woke up a little more comfortably. I recognize that alcohol too late in the day will affect my sleep (and too much will give me gastrointestinal discomfort), but it also greatly decreases my anxiety, which I appreciate a lot and use it intentionally for. Adding it back in reminded me of these things, and I think will help me keep moderation in mind going forward.
(1) The specific reason I decided to do this experiment now is because I had a couple nights of crazy bad Restless Leg Syndrome pains, and attempted to tackle on every single recommendation(2) in a desperate attempt to get a good night's sleep. So no caffeine, no alcohol, normal amounts of exercise with gentle stretching before bed, multivitamin with iron, supplements with calcium and magnesium, extra banana every morning for potassium, staying well-hydrated, and eating well. Nothing changed for about two more nights of really awful leg pain, then it gradually decreased after I put more blankets on my bed to add weight and hasn't been back (much) since, even after going back to my usual habits, so overall I'm calling this a failed experiment since I can't tell if anything I did affected it. Bleh.
(2) Well, the sensible recommendations. I am not sleeping with a bar of soap in my sheets, TYVM.