it's your sister Cheryl!
I work for Petersen Games. AMA
Yes.And since you get Power from it in the Doom phase, it can be used as your final 2 or 3 power to simply transfer that power to your next turn, as if you had captured that many cultists!
In general, any restriction on an ability will be called out in the text for that ability. An example is Thousand Forms, which explicitly states that it can only be used once per Action Phase.
Sleeper doesn't really have power gaining abilities. He only has Power Saving and Power Transferring abilities. Lethargy is an amazing power management tool, but technically adds zero to his power total. Burrow, likewise adds nothing, but does save him power.
However, he can use Capture Monster, and Ancient Sorcery to transfer Power to the following Round in a more powerful way than everyone else - the others can only rely on Capture Cultist, really. When used properly, Sleeper can have a magnificently larger powered turn in mid to late game.