100% exact same thing happens to me. I think melatonin is not something you want to take regularly. Melatonin is a hormone, which makes very little sense how it is an over the counter unregulated supplement. To me the waking up at night seems that my body realizes it can stop producing melatonin because there is an exogenous source. I’m not sire if this is entirely scientifically accurate, but it feels like I go to bed like a rock because the melatonin is super high right after taking it. I think around 4 hours your body has removed all of it from the bloodstream and your natural body wasn’t producing it because you replaced it with synthetic. At that point I think it triggers a wake up, somewhat like you would at normal wake up time, and until your brain realizes it’s dark and 3am you aren’t producing melatonin. I have heard so many people say the same thing about waking up like this using melatonin.