How does sleep work in magic the gathering?

Answer 1

You choose a player to use it on when you cast it. When it resolves, tap all their creatures. When that player next has an untap step, he may not untap any of these creatures. A creature played and tapped after Sleep resolved, may untap in the next untap step. However a creature that was already tapped before Sleep resolved, may not untap. That's because Sleep's effect was applied to it (it affects all their creatures, not just untapped ones), even though it could not physically become tapped by Sleep.