Depends on the method of inflicting it. Risks missing with less accurate options like Sing. Causes you to lose a turn, rather than your opponent. To look at it a different way with Sleep Powder:
- 25% miss chance. Results in -1 action for you.
- 75% hit chance.
- 33% chance of 1 sleep turn. Cost you an action to make them lose an action. No net change.
- 33% chance of 2 sleep turns. Spent one action to stop two of theirs. +1 action.
- 33% chance of 3 sleep turns. +2 actions.
Expected value of Sleep Powder: (.25 * -1) + (.75 * .33 * 0) + (.75 * .33 * 1) + (.75 * .33 * 2) = 0.4925 extra actions per Sleep Powder use. Not bad.
Compare this to something like Swords Dance. Normally deals 100 damage, for ease.
Turn 1: 0 damage.
Turn 2: 200 damage.
Turn 3: 200 damage.
Turn 4: 200 damage.
Racked up 600 damage instead of 400 across 4 turns. Averages out to 0.5 extra actions over your opponent every turn, in a sense, albeit at a 4 turn commitment. Increases in value beyond that, of course.
May be a little more simplistic than reality. Is not a competitive player. Recognizes the power of Swords Dance potentially robbing actions via knockout. Did not acknowledge the momentum swing of an easy switch or Leftovers healing either. Seems relatively okay for Sleep Powder, at least.
Becomes overpowered when things like Spore enter the conversation. Incurs a high stat cost for a reason. Saddled Shiinotic with 405 base stats (with a halfway decent spread). Created more counterplay these days than past days, however: Safety Goggles, Electric Terrain, Misty Terrain, Grass powder immunity, and various abilities. Is that enough? Shrugs. Fights it so rarely, not including that awful 6-star Tera Raid Amoongus.
Suspects some of the reason for the Sleep Clause is for swingyness. Potentially wins on pure luck. Bans moves like Guillotine and Minimize on similar grounds.