I find caffeine, in general, problematic. Some comments:
I think people respond differently to it. I don’t know why that is.
For example, tons of people talk about needing caffeine to “wake up”, think clearly, get moving, etc. in the morning - which I’ll summarize with the term “morning energy”. I’d guess that morning energy is the #1 reason people cite for caffeine consumption. But I have tried different caffeine amounts (though never in doses exceeding ~200mg/day), for weeks or months at a time, and as far as I know I have never once experienced that effect.
If I’m avoiding caffeine as much as I can, when I wake up I get moving pretty much right away.
If I’m on some level of caffeine, when I wake up I get moving pretty much right away just the same. If I don’t have my usual caffeine dose for EX: 3 hours after I get up, it’s not a problem at all for me. I don’t feel more alert or clearer thinking after having the caffeine than I did before. If I have significantly more caffeine than I’m used to I can feel “jittery”, but that’s not a good thing / not a help.
Bottom line, I am pretty sure that caffeine is zero help to me in the morning whether I’m regularly having a significant amount of it or not. It’s possible all the people saying caffeine helps in the morning are lying, or having some kinda placebo effect, or something like that. Or it’s possible there’s some energy effect on me I’m failing to perceive. But I doubt it. I think caffeine affects other people differently than me wrt morning energy.
Caffeine will keep me up if I have a lot of it late in the day. I think that’s pretty standard.
More insidious though, caffeine will wake me up and keep me from going back to sleep as long as 12 hours after consumption. It stays around longer than drugs with pro-sleep effects (like benedryl). For example I’ve had a caffeinated dessert like tiramisu at dinner, taken a benedryl 2 hours later before getting into bed, and predicatably the benedryl wears off about 4-5 hours after that, the caffeine wakes me up and I can’t go back to sleep. So desserts like tiramisu I have learned not to have at dinner.
An effect I have that I don’t hear other people talk about much is caffeine sensitivity. I have, at times, attempted to maintain very low caffeine intake for months at a time. When I did, I found I became very sensitive to relatively small amounts of caffeine. One problem is foods I ate irregularly (like chocolate) would start to affect my sleep. Another problem as you (ET) said is that “decaf” means different things & different amounts than you might expect. And another problem is some recipes have more caffeine than you’d expect for the same “thing”: for example some chocolate cake recipes use espresso as an ingredient! And some flavors of ice cream etc. When I was trying to have low caffeine intake I ended up with what seemed to me like random sleep disturbances. I’d find myself awake at 3am, unable to go back to sleep, and in reviewing the previous day would guess that the cause was the chocolate, or the “decaf” tea that wasn’t as decaf as I thought. That could be misattribution but I don’t think so. One reason is that it stopped happening as soon as I started deliberately getting some caffeine again.
I think, for me, around 100mg of caffeine sometime in the morning results in me sleeping better at night. I wake up less or, if I do wake up, go back to sleep easier. The form doesn’t matter. I’ve tried coffee, black or green tea, chocolate covered coffee beans, and caffeine pills. All seemed to have approximately the same effect, although it takes 2 cups of tea, and even one cup of light roast coffee makes me feel jittery. I’ve heard light roast has more caffeine, so I think that’s why and I avoid it.
But even that isn’t great. After a while (1-2 months) the effect diminishes and I start sleeping worse. I could up the caffeine at that point and probably get the effect back, but I already know that’d be an ultimately fruitless approach. So instead I taper off for a week or so. The chocolate covered coffee beans are good for that, as I can just gradually reduce the amount. If I reduce too fast I get headaches. Once I’m off then I can stay off for a while before the sensitivity comes back, but my sleep isn’t as good.
Anyway, I think if it was reasonably convenient to get to and stay at zero caffeine that’d be best. Since it’s not, I think the next best option for me is to be relatively deliberate about consuming an intentional amount.