What's a refusal present tense?
There are no "reflective verbs" in English like you have in French.The verb "se coucher" is "to go to bed" in English. Your sentence can be translated into different English sentences based on the context.
1.If you are talking about your bed time every day or usually, then you will use present simple:
I go to bed at 8 o'clock.
2. If you are talking about tonight or tomorrow night or any other future time, your would say:
I'll go to bed at 8.
or:
I'm going to bed at 8.