Normally when we sleep, the body goes into a state of tamas and with it, the consciousness loses the thread of the progress of the preceding day. While the progress is not ‘lost’ in the long run, it does mean that we tend to have to re-establish what was done previously time and again. Something similar is said to happen in rebirth, that the being, no matter how advanced, has at least some remedial work to do to get back to the final stage prior to passing from one body to the next. The deepest consciousness uses the time of sleep to move off into other planes and experience things there while the body recuperates its energy and renews its sense of well-being. When we awake there is generally a feeling of starting over that encapsulates the sense that the mood, energy, aspiration, dynamism of the prior day has disappeared. This is one reason why those who undertake spiritual practices sometimes try to go to the extremes of pushing away sleep and trying to remain awake and focused, although, as noted previously, such a process tends to fail and actually can increase the tamasic feeling in the body over time.
It is possible to establish a yogic discipline of preparation for sleep and remaining fixed in the aspiration, to eventually find ways to overcome this slipping back and even turn sleep into a state of yogic progress.
Sri Aurobindo observes: “The consciousness in the night almost always descends below the level of what one has gained by sadhana in the waking consciousness, unless there are special experiences of an uplifting character in the time of sleep or unless the yogic consciousness acquired is so strong in the physical itself as to counteract the pull of the subconscient inertia. In ordinary sleep the consciousness in the body is that of the subconscient physical, which is a diminished consciousness, not awake and alive like the rest of the being. The rest of the being stands back and part of its consciousness goes out into other planes and regions and has experiences which are recorded in dreams….”
“At night when one sinks into the subconscient after being in a good state of consciousness we find that state gone and we have to labour to get it back again. On the other hand, if the sleep is of the better kind one may wake up in a good condition. Of course, it is better to be conscious in sleep, if one can.”
Sri Aurobindo, Integral Yoga: Sri Aurobindo’s Teaching and Method of Practice, Chapter 10, Difficulties in Transforming the Nature, Sleep, pp 311-314