Sleep or nidrA in Sanskrit must be understood thoroughly by the sAdhaka in the course of practice. Usually sleep is thought of as a time to rest the body so as to rejuvenate for the next day. Yoga, VedAnta and tantra, classify the three states of mind as waking (jAgRt), dream (svapna) and deep sleep (sushupti). Usually the focus of life is on the waking state, with little knowledge or attention to the dream state, and even less of the state of deep sleep. However, in Yoga, VedAnta and tantra, sleep has great significance since in that state the individuated Consciousness (jIva) is closest to its Source. There is an interesting discussion comparing modern and traditional understanding of sleep (Chapter 5 The Secrets of Sleep) in SwAmi RAmA's book Yoga and Psychotherapy.
In the Yoga SUtra, sleep (nidrA) is merely another vRtti (pattern, or fluctuation) of the mind. The characteristic of this vRtti is that it is free from concepts or ideas (pratyaya abhAva). (Here nidrA refers to sushupti and not to the dream state where there are objects, thoughts and concepts in the mind.) VedAnta emphasizes the distinction between the dreaming and deep sleep states, especially to show how objects and experiences in the dream state are found to not have the same reality in the waking state. Using this as an analogy, it is explained how the Reality experienced in the fourth state (turIya) renders experiences in the waking, dream and deep sleep states as unreal.
As sAdhanA progresses and once kuNDalini is awakened and rises stably to the AjnA cakra, several interesting discoveries follow. Knowing the distinction between mind and Consciousness that comes after the mind is fully penetrated through samAdhi is a very key step in this process. Then the sAdhaka becomes aware of the movement of the locus of Consciousness as one moves between the states of waking, dream, and deep sleep. It is then that the meaning of the practices of meditation and yoga nidrA become clear.
In the waking state when Consciousness is directed outwards through the mind to the sense and action organs, it appears that objects in the external world are seen, apprehended and manipulated. In this ordinary waking state the mind is like a prism that refracts the pure light of Consciousness. This is why prANa, which is another name for Consciousness in motion, is called so: prANa = pra (first) + ana (eater), which tells us that prANa is the first eater of food. In the Upanishads prANa is called the first eater of food, where by food is meant the mind (rayI). In the tAntric text Shiva Svarodayam, prANa is associated with the sun and it feeds on the mind that is associated with the moon. Here prANa is not restricted to prANa vAyu alone, but it refers in general to Consciousness in motion that flows through the mind and sense organs to apprehend the external world. However, prANa vAyu is also sometimes referred to as prANa because it is the chief among the five principal prANAs viz., prANa, vyAna, apAna, samAna, udAna.
In meditation, through the practice of pratyAhAra, the outflow of prANa through the sense and action organs is withdrawn and through the the process of upward traveling (Urdhvaretu), the entire power of Consciousness is focused at the AjnA cakra. The process of meditation also transforms the mind from a refracting prism to a converging lens. The mastery of samAdhi and samyama enables the sAdhaka to deploy the focused energy of Consciousness through the one-pointed mind in samAdhi on to various levels of reality as perceived by the mind. When the focal point is on the conscious mind with gross objects and subsequently their absence, it is called savitarka and nirvitarka. As the focal point is withdrawn into the unconscious mind that contains the subtle objects encountered in the dream state, it is called savicARa and nirvicAra samAdhi. When the focal point is withdrawn further into the subconscious mind of latent impressions, then the state of prajnA is attained. This is part of the causal body and the state of mind in deep sleep, and constitutes avidyA (nescience).
In the early stages of sAdhanA, the activation of prANa through prANAyAma can cause some trouble with sleep patterns for the aspirant. These are not a cause for serious concern provided the sAdhaka understands their origin and takes appropriate steps. In the early stages of sAdhanA the practitioner has not yet gained control over the flow of prANa, either in the body or through the mind. The reason caffeine and other stimulants create difficulties in sleeping is that they overstimulate prANa that then eats the mind more vigorously. Unfortunately the beginner in meditation does not have control over the mind either, and so there arises a frantic overactivity of the mind that prevents sleep. The same situation can also be experienced in high fever, where the overstimulated prANa can cause a state of delirium in the mind.
To overcome the problem of sleeplessness in the early stages of practice, it is first useful to take the obvious steps of going to bed early and at the same time every day. Half a glass of warm milk can be useful. It is not good to focus on work or to meditate just before bedtime. If these obvious and simple remedies do not work, another alternative to counter this problem is to learn the practice of yoga nidrA.
Yoga nidrA is a companion practice to meditation and is very well explained in SwAmi JnAneshvara BhArati's website. Yoga nidrA is practiced in shavAsana (corpse pose). In yoga nidrA the locus of Consciousness is moved to the hRt (heart) cakra through first making the mind one-pointed by the 61-points practice. The culmination of this practice is to now gently bring the one-pointed mind to rest at the hRt cakra. Now that Consciousness is located in the hRt cakra, which is its usual locus at the time of deep sleep, the mind is slowly emptied of thoughts to bring it to a state of pratyaya abhAva (absence of thoughts or concepts). This is the counterpart of meditating with Consciousness at its locus in the waking state. In yoga nidrA there is no sharp focus of the mind on objects, but rather it is made blank. It is different from sleep because there is complete awareness of the mind in its deep sleep state. It is an excellent technique to observe and attenuate the samskAras and deep vAsanAs in the subconscious mind. However, the beginning aspirant can use it as a technique to fall asleep if there is difficulty sleeping. The only point to keep in mind is that in the later stages of practice the sAdhaka will have learn that one should not fall asleep in yoga nidrA!
In the advanced stages of practice the yogi finds that less sleep is needed because the mind has become cleared of many samksAras and so the time spent in dreaming is greatly reduced, or practically eliminated. Through the regular practice of yoga nidrA the yogi needs very little sleep. Sometimes 3 to 4 hours is sufficient for the mind to be rested. Then is glimpsed the true meaning of the verse in the Bhagavad GItA:
ya nishA sarva bhUtAnam tasyAm jAgarti samyamI
yasyAm jAgrati bhUtAni sA nishA pashyato muneh (GItA, II.69)
That which to all creatures is night, is where the man of self-control is wide awake,
and that in which all creatures are wide awake is night to the sage who sees.
This does not mean that the beginner sAdhaka should sit up bleary-eyed through the night and sleep in the day! On the contrary, it is a state that emerges naturally as the mind is progressively purified. When the unreality of the external world is perceived more and more clearly, the waking state becomes less and less attractive and real to the yogi. On the other hand, when at night Consciousness is closest to its Source, the yogi becomes more and more awake to Reality. This is also the reason why the Shiva Svarodayam recommends that the yogi should flow the piNgala (solar) breath in the night and the iDA (lunar) breath at night. The solar breath is more active while the lunar breath is more calming. As the true nature of Reality is progressively unveiled, the yogi becomes less influenced by the flow of PrANa (HiraNyagarbha) in the external world, and becomes resistant to the tamas that envelops other beings at night.