“Q: What, then, is the process of going to sleep?
A: This is partially explained by Physiology. It is said by Occultism to be the periodical and regulated exhaustion of the nervous centres, and especially of the sensory ganglia of the brain, which refuse to act any longer on this plane, and, if they would not become unfit for work, are compelled to recuperate their strength on another plane or Upadhi. First comes the Svapna, or dreaming state, and this leads to that of Shushupti.
Now it must be remembered that our senses are all dual, and act according to the plane of consciousness on which the thinking entity energizes.
Physical sleep affords the greatest facility for its action on the various planes; at the same time it is a necessity, in order that the senses may recuperate and obtain a new lease of life for the Jagrata, or waking state, from the Svapna and Shushupti.
According to Raj Yoga, Turya is the highest state. As a man exhausted by one state of the life fluid seeks another; as, for example, when exhausted by the hot air he refreshes himself with cool water; so sleep is the shady nook in the sunlit valley of life.
Sleep is a sign that waking life has become too strong for the physical organism, and that the force of the life current must be broken by changing the waking for the sleeping state.
Ask a good clairvoyant to describe the aura of a person just refreshed by sleep, and that of another just before going to sleep. The former will be seen bathed in rhythmical vibrations of life currents – golden, blue, and rosy; these are the electrical waves of Life.
The latter is , as it were, in a midst of intense golden-orange hue, composed of atoms whirling with an almost incredible spasmodic rapidity, showing that the person begins to be too strongly saturated with Life; the life essence is too strong for his physical organs, and he must seek relief in the shadowy side of that essence, which side is the dream element, or physical sleep, one of the states of consciousness.”
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 