If he didnt wake up after a night out, we can assume it was likely to be drugs related.
Sorry mate, but could be anything. My uncle died in his sleep, with no warning ad most probably no suffering, brain annurism. He knew he had it, but many who have them, don't know - he had epilepsy though and because of this they were not able to operate on his brain to remove it, because of the risk of seizure during the operation. He was essentially left living with a ticking time bomb.
My uncle hadnt been drinking the night before (he wasnt a drinker at all due to epilepsy and medication), but he could have been and simply not woken up the next morning. So its quite incorrect to assume anything on the basis of him dying whilst asleep.
Could be the result of a medical condition, whether known or unknown, and it isnt all that rare for people's bodies to simply pass away without just cause either.