I had never had a similar problem before.
Today, all of a sudden, it takes from 1 to few seconds - no more than 4 - to crash after waiting, resting or sleeping. Not while. After.
It always does that on those occasions and ONLY in those occasions - ok it can do it after extensive amounts of time played, but that was normal and has always occurred.
As an additional note, the crash occurs with every save file now, and in one instance it also started to lag badly, but reloading could solve the latter.
I have autosaves disabled, and I have not changed anything to Oblivion or my pc.
I do hope it is just because my pc had been on for many hours.