Completely GM call. There is nothing to indicate that you can't.
If you decide that you can, then:
Net Hits 1-2 -> You get a glimpse at what they are dreaming about. Perhaps you can have guided somebody through 'hypnosis-type measures to be dreaming about what you want them to be dreaming about. Maybe it's random. Up to the GM.
Net Hits 3-4 -> Works as normal.
Net Hits 5+ -> works as normal.
There is nothing anyone here can say with any level of definite clarity that it would work one way or another, either through fluff, RAW, or their understanding of how brain's work and what happens when we sleep.
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What is the benefit of doing it while they sleep? That they don't know they were probed? Whether you can or can't the subject will know that they have been probed, though they may not know the source, this is black and white in RAW. The feeling of magical brain invasion is real. But RAW already allows that they may not know the source, so it doesn't add any real benefit. You can as easily Mindprobe them from invisibility or any number of things.
Does it wake them up? Maybe, maybe not: GM call. Waking up from sleep or being knocked out with a definite feeling you've been mentally assaulted can make for as fine a story as any other.
What is unbalanced by allowing this? You have the subject. You can wait till they wake up and then Mind Probe them from invisibility. The end result is the same. They know they've been probed, they don't know the source.
I guess mostly it comes down to how do you feel about it for your story purposes and if the tables were turned and in other circumstances (so you can make a consistent ruling).