That point about the placebo effect is spot on. Our brains are wired to expect caffeine, so when we switch to decaf, we might feel tired just because we think we should. For example, if you usually have coffee at 8 AM, your body gets ready for that boost. Without it, you could feel sluggish even if the decaf has a little caffeine. It's like when you drink fake beer at a party and still act a bit tipsy (your mind plays tricks on you). So yeah, it might not be the beans themselves but what we believe about them.
Consider how this plays out with other daily habits too. Your body learns the pattern of your morning routine, so skipping the real coffee disrupts that whole rhythm. I've seen friends swear they get a headache from decaf, even though science says there's barely any caffeine left. It shows how powerful our expectations are in shaping what we actually feel.