Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 3
These thoughts that attack in sleep or in the state between sleep and waking do not belong to any part of your conscious being, but come either from the subconscient or from the surrounding 
atmosphere through the subconscient. If they are thoughts you had in the past and have thrown out from you, then what rises must be impressions left by them in the subconscient – for all things thought, felt or experienced leave such impressions which can rise from there in sleep. Or the thoughts can have gone out from you into the environmental consciousness, that is, an atmosphere of consciousness which we carry around us and through which we are connected with universal Nature and from there they may be trying to return upon you. As it is difficult for them to succeed in the waking state, they take advantage of the absence of conscious control in sleep and appear there. If it is something new and not yours, then it can be neither of these, but an attack of some outside Force.
It is to be hoped that as you have rejected them, they will not come again, but if they do, then you must put a conscious will before going to sleep that they should not come. A suggestion of that kind on the subconscient is often successful, if not at once, after a time; for the subconscient learns to obey the will put upon it in the waking state.