Sleep is a natural, regularly occurring condition of generalised rest and relaxation of the mind and body, characterised by the absence of conscious thought, sensation or movement. Sleep is a natural form of pratyahara (sense withdrawal) which occurs as our consciousness disassociates itself from both the sensory and motor channels of experience. When the consciousness disassociates itself from the sensory organs (gyanendriyas) and the motor organs (karmendriyas), contact between the sensory/motor cortex of the brain and the external world is gradually lost. As this occurs, the consciousness is progressively withdrawn and redirected internally towards its source.
Research has shown that, during the descent into sleep, the sense modalities are disengaged systematically in a fixed order as awareness is internally directed towards the deeper levels of the mind. According to tantric philosophy, this entry into sleep can be understood as the progressive withdrawal of the awareness back through the chakras towards its cosmic source (sahasrara).
For example, according to research, the sense of smell (olfaction) is the first sense to disengage. In tantra, this corresponds to mooladhara chakra and the earth tattwa or element. Smell is followed by taste (gustation), corresponding to swadhisthana chakra and the tattwa of water. After taste, visual capacity, which is the modality (tanmatra) of manipura chakra, disappears, then touch (anahata, air element), and finally hearing (vishuddhi, akasha or ether element). Therefore, the yoga nidra state, in which awareness of verbal instructions alone remains, corresponds to a stage of sleep at the very borderline between wakefulness and dreaming.
Recent French research has revealed that when we sleep, our consciousness moves through at least four distinct stages. These stages follow one another at variable time intervals in cycles of approximately 90 minutes, which continue throughout the sleep period.
At the onset of sleep, there is a stage of deep relaxation and altered consciousness (hypnagogic period), which again occurs during the final emergence from sleep (hypnapompic period). In an untrained person, this lasts for only 5% or less of the total sleeping time, but it can be developed and extended by the practice of yoga nidra. During this transition period, alpha waves (frequency 8-12 c.p.s.) are produced by the brain.
During stage 2 sleep (dreaming sleep), the brain emits slower rhythm theta waves (frequency 4-7 c.p.s). The first two stages have been together termed the REM (rapid eye movement) phase of sleep by earlier researchers. This is a period of intense psychic activity, and is far from the restful and inactive state that most people think of as sleep. During this period, pent-up tensions which have accumulated in the subconscious mind during the working hours, are released in the form of an internally projected 'dream dialogue' which is visually witnessed by the subject. Not only do the eyes move in a rapid, random, continuous way, following the dream imagery which is occurring, but whole body movements are common as well. The physical body may move 30-60 times or more during REM sleep, sometimes even suddenly or violently. It is during this phase that phenomena such as sleepwalking (somnambulism) and trance states occasionally occur.
Stages 3 and 4 of the sleeping cycle correspond to deep (dreamless) sleep, when slow delta rhythm waves (0-4 c.p.s.) are produced by the brain. During this stage, the unconscious mind, source of instincts, drives and deeply buried experiences of earlier evolutionary stages, manifests. In contrast to the dream state, all mental activity and fluctuation disappears during deep sleep. The samskaras (past impressions) and vasanas (latent desires) are inactivated and the mind and the body are paralysed. Consciousness and prana alike withdraw from the individual body and mind and retreat towards the unmanifest creative source. This period is known in the tantric and yogic scriptures as 'the night of Brahma' and also as 'the womb of creation' (hiranyagarbha).
Scientific investigators have concluded that sleep is a complex process involving specific neuro-physiological and psychological events, which mutually influence each other. Therefore, the quantity and quality of our night's sleep can be affected by either one of these factors. For example, insomnia, or the inability to sleep, can be due to either physical or psychological causes, or to a combination of both.
Recent studies have shown that often, people complaining of insomnia really do sleep, but suffer from a delusion that they have lain awake all night tossing and turning. In these experiments, subjects were monitored on an electroencephalograph (EEG) throughout the resting period. According to the delta wave patterns emanating from their brains, they were actually experiencing deep sleep. This type of insomnia is psychological, and can definitely be alleviated by the practice of yoga nidra.
However, in some cases, failure to sleep is not a mental, but primarily a physiological reality. For example, if there is disharmony in the secretion mechanisms of the endocrine glands due to emotional turbulence, insomnia may develop as a physiological response to raised hormone levels in the blood. Stimulants like tea or coffee also disturb sleep patterns by exciting the reticular activating system in the brain and creating activity in the coronary circulation and behaviour.
In order to correct such physiological insomnia, the precise cause must be identified. Where hormonal hyperactivity is responsible, yoga training, including yoga nidra, will correct the problem gradually. In other cases, a simple change in exercise and eating habits and times may be all that is required.
Sleep can be induced in two distinct ways: biologically or psychologically. In the first case, one can take sedatives, tranquillisers or hypnotic drugs, which act upon the brain's central mechanisms, inhibiting the cerebral cortex and the reticular activating system in order to depress the level of consciousness. Sleep inevitably follows, but sometimes it is not of a restful or natural quality. The brainwave patterns are often disturbed and the natural sleeping cycle is not established. Long, unpleasant dreams, mental haziness and depression, and even over dosage, can be the result.
Some people who suffer from insomnia will not use sedatives because they know that this system resists or develops immunity to them after some time. They are aware that all the different types of sedatives and hypnotics have detrimental long term side effects. Therefore, they try to find another autonomous method which can induce sleep.
Yoga nidra is such a method, and it is extremely powerful, as many former sufferers from insomnia can attest. When a person practises yoga nidra, he develops the capacity to induce and control sleep through mental processes. He makes the conscious decision to regulate his biological functions through his own mental efforts.
In both of these methods, sleep is not voluntary but is artificially induced. In drug-induced sleep, the biological processes of the brain are chemically influenced, and this has an indirect influence on the mind. However, in yoga nidra, first the mind is influenced directly, and as a result, the necessary biological responses take place in the brain. Both are effective ways of inducing sleep, but there is one important difference. In the first case, you are becoming dependent on the existence of exogenous chemical agents whose long term effects are still in question. In the second case, you are gaining an invaluable tool for healthy and successful living which can lead you to higher awareness.
Sleep requirements and habits vary from person to person, and no fixed rule can be applied in this matter. However, 6-8 hours is a maximum, for beyond this, extra sleep becomes detrimental. Most studies have reported that the most efficient sleep is that gained before midnight, and that the optimal sleep period is 4-5 hours. However, if we are tense, we may require 6-8 hours, or even more.
An individual's precise sleep requirements really depend on the level of tensions accumulated during his daily activities. If a person is involved in yoga and spiritual practices, especially yoga nidra, meditation and pranayama, and is relatively free of tensions, 4-5 hours sleep is enough. However, for those who do not follow any spiritual discipline, and whose minds are overloaded with thoughts of business, family, property, etc., a little more sleep is necessary.
The French report reveals that we invariably sleep deeper in the initial 2 hours than in subsequent cycles, with less dream sleep intervening. Persons who sleep less spend more time in stages 3 and 4, while the time spent in the REM phase (intense psychic activity, stress release and dream consciousness) increases with the duration of the sleep period. In the initial cycle, dreaming may occupy only 20% of the total time period, with 75% devoted to deep sleep. By the 3rd or 4th cycle, however, dreaming time may rise to 50%. The proportion of less efficient sleep increases markedly in the latter cycles of any sleep period, especially in extended periods of slumber.
Sleep initiates changes in most of the systems of the body. The withdrawal of sense awareness is accompanied by a deeper, more regular respiration pattern, with exhalation becoming longer and less oxygen being consumed. Simultaneously, the heart rate becomes slower and the metabolic rate drops.
Researchers have also been able to record the release of muscular tensions as the awareness enters the sleeping state. Using the electro-myograph (EMG) they have demonstrated that the physical body relaxes in distinct stages. Release of tension is recorded initially in the large postural muscles of the spinal column, back, thighs, legs and neck. Next to 'let go' of their resting tension level are the smaller intrinsic muscles of the hands and feet, followed finally by the fine muscles responsible for facial expression. As progressive muscular relaxation occurs throughout the entire musculoskeletal structure, the various body tissues begin to spontaneously liberate toxins and metabolic wastes into the circulation. It then takes some time before they are completely eliminated from the body.
At the psychological level, going to sleep involves dissolution of the wakeful ego state. This is accompanied by loss of the normal sense of reality. The untrained person loses his grasp upon the concrete external reality during the short 3-5 minute twilight phase which he traverses on the border between wakefulness and sleep. It is replaced almost immediately by the 'dream reality', in which another restructured ego operates without awareness of the parallel existence of the waking reality and its ego.
It is as though we are playing two distinct roles, acting in two concurrent dramas, which are being performed in adjacent theatres. Each time one session is over we rush quickly to the second theatre in order to fulfil our role there. However, each time we change theatres for the next performance, we experience a bout of amnesia and forget the role we have just been playing. Only under exceptional circumstances do the twin realities in which we are acting become known to one another. This occurs, for example, when we happen to remember a vivid dream upon waking up. However, for the most part we remain unaware of our engrossing twin commitment to two, and perhaps more distinct realities.
Only the briefest of interludes (5% of total sleeping time) is spent in shuffling between these two pageants. As long as we continue to mechanically cross this curtain between waking and dreaming states without awareness or recognition, we are destined to remain ignorant of our real nature.
Yoga nidra has been devised to lift this veil, so that we can directly experience the underlying consciousness, in which the individual mental fluctuations and modifications (vrittis) such as wakefulness, dream and deep sleep are ever manifesting, like the changing scenery observed from a train window. Only when we can see and witness the various dimensions of our mind objectively, traversing the barrier between the individual states while preserving our awareness intact, can we catch a glimpse of what we really are. This is why yoga nidra is the portal to the greater knowledge and true wisdom lying within. Sleep (nidra) is ignorance but yoga nidra is the doorway to higher consciousness. It is the means of understanding, integrating and utilising the various dimensions of our own mind.