Oracle Text

Tap all creatures target player controls. Those creatures don't untap during that player's next untap step.

Card Rulings

10/1/2009 The second part of Sleep’s effect affects all creatures the targeted player controls as Sleep resolves, not just the ones that Sleep actually caused to become tapped.
10/1/2009 Sleep tracks both the player and the creatures. If one of the creatures controlled by the targeted player as Sleep resolves changes controllers, the creature will untap as normal during its new controller’s next untap step.