Substance use disorder occurs when a person needs alcohol or another substance (drug) to function normally. Abruptly stopping the substance leads to withdrawal symptoms. Addiction means that a person has a strong urge to use the substance and cannot stop, even if they want to.
So it is important to maintain positive nitrogen balance in the body while sleeping. When you do and conditions are right so that your rate of protein synthesis exceeds your rate of degradation, and you maintain a positive nitrogen balance, you can grow while you sleep.
REM sleep is an active mind inside a paralyzed body, while nREM sleep is a resting mind inside a functioning body. This is why sleepwalking is possible during nREM sleep and not during REM sleep: your body is paralyzed during REM sleep but not in nREM. As for why sleepwalking occurs... well, we still don't really know. See Wikipedia: Sleepwalking for much of the current knowledge.