That's just your unfortunate luck really. Sleep turns are usually never for that long and relying on a move such as Hypnosis isn't a very good option either, seeing as it firstly has a rather shaky accuracy. Despite you being able to boost this with a Wide or Zoom lense ( recommended over the Wide Lense seeing as Musharna is disgracefully slow) none being recommended in general however, it still really isn't worth it, because Dream Eater Musharna is in general really not that good of a Pokemon.
If you would want a more succesful offensive Musharna that could last awhile, I would rather recommend something such as bulky CM
Musharna @ Leftovers
Ability: Synchronize
EVs: 240 HP / 252 Def / 16 Spe
Bold Nature
- Calm Mind
- Psychic / Sotred Power
- Moonlight
- Baton Pass / Shadow Ball / Signal Beam
Or a bulky Pivot:
Musharna @ Leftovers
Ability: Synchronize
EVs: 240 HP / 252 Def / 16 SpD
Bold Nature
- Moonlight
- Thunder Wave
- Baton Pass
- Psychic
Both which I would honestly think would work much better for you than Dream eater