system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can a nicotine (and possibly other substances) addicted brain sleep without having to get a 'fix'? During the day it's a lot harder to 'stay clean' for 10 hours.

Little side question: Would be interesting to know if all addictions are at rest during sleep. Personally I don't have the experience besides nicotine and maybe caffeine.
gpt
if you are addicted to a substance enough, it will affect your sleep. it may be minor, waking up a time or two and drifting back off, or it may be severe, waking up several times and not being able to go back to sleep. sleeping does the same thing going without any other time does. it starves your body for the substance. that is why most users will fix right when they get up. even if it is nicotine or caffeine, you need it right away.