At the core, we are pure awareness devoid of any object (thoughts). We can direct this awareness which we call the art of attention. We're pure awareness and we decide to attend something. However, to attend and receive information from the physical plane, we need appropriate instruments. Brains and its organs of senses are those instruments. If these instruments have fault, obviously we as awareness don't receive enough information and it seems our consciousness is reduced. In waking state, we receive info from physical senses. In dream state, these senses are in suspended mode but mind is active in imagining the experiences in a virtual world and hence awareness has an instrument. In deep sleep, even mind is in a state of rest and hence awareness has almost no active instrument. Still, we awareness does exist and thus we know that we had a good or bad sleep when we wake up.