You know, the same way one might forget to eat food. I keep a bottle of migraine zappers for this exact occasion when I wake up in the middle of the night with my head exploding and I’m like “oh. right. I’m dependant on a substance. nice fumble, dweeb”
Yeah I get a headache that night if I miss the late morning caffeine dose. I find they are really resilient and distinct headaches though, so I will usually try to have a little caffeine with the painkiller, like strong tea or milk with coffee in it. It doesn’t mess with my sleep and works much faster and more thoroughly than a pill alone.