go to sleep? ("I will sleep at [time]" doesn't make a whole lot of sense in my dialect of English at least.)
I think the issue here is that the Hindi sentence is a bit odd/unnatural, and then the English translation is trying to be super literal so we can learn the grammatical constructions (as opposed to learning natural speech). Does that makes sense?
To say I will go to sleep, you can say मैं सो जाऊँगा. However, I think that construction is too "advanced" for the current DL Hindi tree. I'd guess that if they wanted to teach "go to sleep" they would use that, but all they wanted to do was teach the basic future tense and they grabbed a verb, "to sleep" and VOILA! : "will sleep" :)