Can hypomania/mania occur without the decreased sleep?

Yes: Lack of sleep can bring a stable perso out of stability but so can a lot of other things like stress or schedule changes, not just sleep deprivation.

Yes, but: Decreased sleep is one of the most typical symptoms of mania and hypomania. Still, every person is unique and what's typical for most may not be true for the individual. Clinical judgement is always required, and knowledge of the entire context.