Gus:
To my greatest surprise, I just found out that it´s incorrect to say: I slept at ten o´clock last night. Instead, we must say: I went to bed at ten last night. However, it´s hard to digest this.... I wonder, better saying, I still wonder why can i not say I slept.....? Is there any plausible explanation, please?

It´s understandable that you get puzzled, Gus.

In the beginning of our English learning, and for quite a while after we get used to it, our brain works as if the grammar is that of Portuguese.

So, it´s a "plain translation" what you are experiencing, as though they had to talk as you talk.

But then, just try to remember about "convention", they say "I went to bed" meaning that "fui dormir".  Or you should think that they don´t sleep at a given precise time as we do...(eu fui dormir/eu dormi às nove horas, dizemos) they will say that "I was/fell asleep around/by nine o´clock last night   etc...").

Portuguese in English is the plausible explanation, then.

That´s why people here say they live in Big Field (instead of Campo Grande), Rio de Janeiro (January River), say their name is Joseph (instead of José) and make other plain explanations.

Try to imagine if my name is Guilherme I have to say to an English speaker that "my name is William?" and to a German I have to say "my name is Wilhelm?"...!  No, if I am a Brazilian I say I am Guilherme, period.