Shavasana provides the most natural (anatomical) and comfortable position of the body which contributes to the physical and mental rest in the first instant, leaving no imbalance in the muscles of the body.

All the joints are consciously loosen.

Horizontal and relaxed position of the body on ground facilitates efficient and easy blood circulation.

Blood pressure and heart rate is reduced and maintained at minimal level because there is no need to circulate the blood with force. 

No system of the body is required to work against the gravity, e.g. flexor and extensor muscles can also relax as there is no need to balance the body against gravity. As a result, the metabolic rate is also reduced. Breathing becomes
slower, slightly deeper, rhythmic and mostly abdominal in nature as one progresses in relaxation.

We are not conscious of this abdominal type of breathing when we simply take rest, but in Shavasana the movements of the abdomen are consciously attended to, at least in the beginning. 

The abdomen moves up when one inhales and sinks down when exhalation occurs.

If one does not find these movements happening, he has to correct his breathing by allowing only the stomach to move up and down and by stopping chest movements voluntarily in the beginning. This is achieved easily if one relaxes the thorax region. 

After a short practice of such abdominal breathing rather consciously, one starts watching these abdominal movements as a third person.

In Makarasana one watches the movement of the lowermost lumbar and upper hip portion along with the breathing phases.

In the abdominal breathing, the diaphragm also moves slowly which has got a soothing effect on the higher brain centers.

One immediately feels calm and quiet. The abdominal breathing gives rhythmic and gentle massage to the abdominal organs, remove their congestion if any and tones up their muscles. It has a beneficial influence over the circulation in the pelvic and abdominal region.

Even before attending to the abdominal breathing in this way, all the body parts and their muscles are relaxed consciously one by one or simultaneously.

We can experience (with the closed eyes) the difference between the tension and the relaxation in the skeletal muscles and joints, e.g. in the hip joints and in the facial muscles and muscles of the lower limbs, etc.  

This sort of a training is required at least in the beginning to relieve muscular tensions as well as to engage the mind properly in such a thing from where new stimulations are not expected. 

Otherwise one undergoes a conscious thought process and brings more tensions through memory, intellect and imagination.

After this one can easily practice pranadharana in which one observes the incoming and outgoing flow of breath passively. One can witness (as a third person) the current of air moving in up to the lungs during inhalation and going out through nostrils again during exhalation. 

Some people feel easier to watch the flow of breath at the tip of their nostrils. Various stages in the technique of Shavasana have been developed upon the principle of progression from gross to a subtler level. 

One is gradually trained to withdraw his awareness from the gross aspect of the body and to develop awareness of more and more subtler functions of the body and mind. For example, after these steps in relaxation as mentioned above, one develops and remains aware of the touch of air inside the tip of the nostrils. 

Then one takes his awareness to a feeling of a minute difference of temperature of inhaled and exhaled air.

One can feel a cooling sensation while inhaling and warm sensation while exhaling at the tip of nostrils. 

As there is least interference from the cortical region of the brain during such practice,

One realises the reduction in number of thoughts entering in the mind particularly when one practices any one of the subtler stages mentioned above. One can easily then develop an attitude of a witness towards one's own thought.

In this way, instead of getting involved in any thought process, one observes or watches them as a third person coming in and going out of the body-mind complex. Rest of the body is forgotten.

The whole body is so loosen that the muscle tone is extremely reduced.

This resolve tensions in the nervous system, calms the mind, making it more and more peaceful. 

One gets gradual training for contemplating and relating mind to infinity as the awareness is withdrawn from all the sensory as well as motor organs (function) and focused on the thoughts.

One experiences a 'thoughtless' condition of the mind since all the analytical activities of the intellect (at cerebral cortex) are stopped. There is no function of interpretation.

This particular training in Shavasana helps the individual to focus the mind on the infinity and to slacken the efforts (motor function of the brain) during the practice of other asanas. 

If Chitta (consciousness) is going to be relaxed in Shavasana, its contact with sensory and motor organs will progressively become weaker and ultimately it could be completely cut off for time-being.

Such functional dissociation of Chitta from organs is termed as Pratyahara. 

It gives a momentary experience of Samadhi to a practitioner. Thus Shavasana also helps a practitioner to achieve a stage of Pratyahara and a higher state of consciousness i.e. Samadhi. 

Even though sedatives and tranquilizers apparently give a feeling of relaxation, it is temporary. 

The chemical action of these drugs is to inhibit or to block the neural mechanisms at some level or the other in the nervous system. 

The mental tensions, worries and anxiety remain as they are and again start working in the same fashion soon after the effect of the drug is over. 

One gets addicted to these drugs and the doses of such drugs go on increasing. This is true to some extent in hypnotically induced relaxation technique also.

In yogic relaxation, the whole approach seems to be different. It allows tensions to come to the surface in an inactivated condition so that they can find their way out without disturbing the individual.

Tranquility and peace of mind that is achieved by a person after the practice of Shavasana is of a lasting nature. 

The practice of Shavasana has been found effective and most suitable in the treatment of hypertensive and insomnia patients since it removes psycho-physiological tensions, reduces anxiety, brings about tranquility, rejuvenates the neural functions, and establishes balance in all the functions of the body. 

It gives rise to the sense of well-being and a mental poise. It is therefore a boon for the patients suffering from high blood pressure. 

This has been proved scientifically all over the world by eminent cardiologists. It also acts as a supportive treatment in various diseases like diabetes, asthma, hyperacidity, arthritis etc.