This. For me, exercise is the gift that keeps giving.

Caffeine works for a couple of days and then becomes a requirement. Any ongoing benefit is difficult to discern.

There are other things that make a huge difference to productivity for me :- - have a plan for the day - make sure to work on stuff that is actually important - look back at the day and see where time was wasted or I went off track and think about how to do better next time - stop working at a fixed time (with a little leeway if I am really in the zone and enjoying it.) This leaves me excited to get to work in the morning. Scheherazade effect :-)

Plus on on this. That first cup of coffee after not drinking it for months is amazing. The 200th? Meh. And the effects become less and less, with the result being that you feel lethargic until you drink it, which then is just a dependency. One problem is that people can't take a 30 minute nap when they need to; I highly urge you to find a work environment where you can do that.