Send to Sleep — ruling 1

If the spell mastery ability applies and a creature affected by Send to Sleep changes controllers before its old controller’s next untap step, Send to Sleep will prevent it from becoming untapped during its new controller’s next untap step.

Send to Sleep — ruling 2

Send to Sleep can target tapped creatures. If a targeted creature is already tapped when the spell resolves (and the spell mastery ability applies), that creature remains tapped and doesn’t untap during its controller’s next untap step.

Send to Sleep — ruling 3

If you chose two targets and one is an illegal target when Send to Sleep resolves, that creature won’t become tapped and it won’t be stopped from untapping during its controller’s next untap step (if the spell mastery ability applies). It won’t be affected by Send to Sleep in any way.

Send to Sleep — ruling 4

Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won’t count because it’s still on the stack as you make this check.

Send to Sleep : règles, fonctionnement et interactions (MTG)

Send to Sleep is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Tap up to two target creatures. Spell mastery — If there are two or more instant and/or sorcery cards in your graveyard, those creatures don't untap during their controllers' next untap steps.

How it works : Send to Sleep

Original English text — Wizards of the Coast / Scryfall

#1June 22, 2015· Wizards of the Coast
If the spell mastery ability applies and a creature affected by Send to Sleep changes controllers before its old controller’s next untap step, Send to Sleep will prevent it from becoming untapped during its new controller’s next untap step.
No other card is named in this ruling.
#2June 22, 2015· Wizards of the Coast
Send to Sleep can target tapped creatures. If a targeted creature is already tapped when the spell resolves (and the spell mastery ability applies), that creature remains tapped and doesn’t untap during its controller’s next untap step.
No other card is named in this ruling.
#3June 22, 2015· Wizards of the Coast
If you chose two targets and one is an illegal target when Send to Sleep resolves, that creature won’t become tapped and it won’t be stopped from untapping during its controller’s next untap step (if the spell mastery ability applies). It won’t be affected by Send to Sleep in any way.
No other card is named in this ruling.
#4June 22, 2015· Wizards of the Coast
Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won’t count because it’s still on the stack as you make this check.
No other card is named in this ruling.