I just know that "I'll go to bed" sounds unnatural but can't tell why. Though, I believe I can use this sentence if I have things to do very early in the morning and think that it would be o good idea to go to bed now to get enough sleep.
But outside any context, does it sound odd to form such a sentence if I've decided to go to bed right at the time of speaking?
I really find it hard to explain the difference between "will" and "going to" at times.