Can hypomania or mania occur without decreased sleep?

Yes: They can be manifestation of sleep related problems or independent of these.

There can be trigger: Other than sleeplessness. Prescription medications are one (antidepressants, stimulants, steroids, decongestants, asthma inhalers ; antabuse) as well as caffeine ; illicit drugs. Lack of sleep, menses, head injury, ms ; infectious diseases are other causes. Psychosocial stressors may include death of loved one, divorce, loss of work, trauma ; financial problems.

Yes: While decreased sleep is a very typical symptom of (hypo) mania no single symptom is absolutely required of the state.

Sleep: Sure, any form of sleep changes can accompany mood changes.

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.