How to say that I (or someone else) wants to do something less (for example I want to sleep less) in Japanese? What about I want to have less of something? (For example I want to eat less, or I want to have fewer problems.)
Literal translations such as 「(もっと/より)少なく + verb + したい」 would sound unnatural... There would be several ways to say that, but off the top of my head right now, I think you could probably use 「～を少なくしたい」 or 「～を減らしたい」, as in:
I want to sleep less. (lit. I want to decrease sleeping time.)
I want to eat less. (lit. I want to decrease eating amount.)
I want to have fewer problems. (lit. I want to decrease problems.)
less is a powerful English word which doesn't have a direct equivalent in Japanese for all cases. It depends on the action you're talking about. For sleep, you could say:
Gloss: Every day - more - shortly - want to sleep - eh
TL: "Man, I wish I could sleep less every day."
Here we see "more + adverb" that fits the verb (referring to time here).
Another way to get around this is to say something grammatically different but functionally the same (re: to have less of something):
Gloss: Clothing - a little - ought to reduce - eh
TL: I should really get rid of some of this clothing. (while inspecting one's overflowing closet, for instance)