Except you've already had 5 hours of your body metabolizing caffeine, so the effects of the remaining half a dose after 5 hours aren't nearly the same as if you took that half a dose after a night's sleep and not having had any. Your body becomes accustomed to it. That's how all drugs work. You can take 6 grams of mushrooms and it'll definitely still be in your system after 6 hours, but the effects won't be the same as taking 3 grams straight up, it will have almost entirely worn off despite the residual psilocybin in your body.