"You never sleep."
Translation:तुम कभी नहीं सोती हो।
For this sentence there are two forms, one for each gender: The one used here is तुम कभी नहीं सोती हो। (Said to a female,) and the other is तुम कभी नहीं सोते हो। (Said to a male.) When speaking to a male, you will always conjugate verbs with ते on the end, the only exception to this is if you are referring to them as तू , in which case it would be तू कभी नहीं सोता है। But you only say तू to people you are very very close with, otherwise it is seen as rude.
Couldn't this also be taken as 'You sometimes don't sleep'? (Which is distinct in meaning from the given 'You never sleep'.)
Would you always use कभी-कभी for 'sometimes' in such cases where it's followed by नहीं + verb, if you meant 'sometimes don't [verb]' as opposed to 'never [verb]'?
Yes, "कभी-कभी + verb" will in most cases replace "कभी नहीं + verb" when you want to change the meaning from "never do + verb" to "sometimes do + verb".
If you wanted to imply the literal meaning of "Sometimes you don't sleep", you would need to say "तुम कभी-कभी नहीं सोती हो।
Or as is more commonly spoken, तुम कभी-कभी सोती नहीं हो।