In deep sleep you are not at the same vibratory rate for them to tune into. Healing has a chance of greater success because the person may be in bed resting in a semi-awake state and receptive to you. By so-called imagination, you can heal someone. There is no such thing as 'imagination.' That word should be thrown out of our vocabulary. If you imagine something you create it. You just don't create it on the physical plane. Before you move your arm consciously, you must think it/imagine doing it. This thinking of doing it is action on the astral plane. The astral arm moves and as a result the physical arm responds. Phantom limb has to do with this as well.
We spend 1/3 of our lives in the astral world/dream world and yet it is given no reality. In my post Thought Mechanics I show what transpires in thinking. The same holds for dreaming. Dreaming is (can be) wordless/happen at enormous speeds - what people call 'deep dreamless sleep' - there is no such thing. In deep sleep we are at a rate of vibration so high we are disconnected with the physical plane. REM does not show up. If a transformer is placed on a table with a penny and it vibrates 50 (Europe) or 60 (US) cycles per second, it buzzes noisily. But if you increase the rate of vibration to one million CPS the penny appears not to move at all.