This asana is sometimes called a "candle" because the body is kept straight as a candle in this posture. The Sanskrit sarvangasana means 'all the body'.
This asana has countless benefits. Sarvangasana activates most of the important glands and help improve their function. This asana is equally important for both men and women. It can be tried by persons of all age levels.
 

Position and Readiness

Lie dawn on your back on the floor. Keep the palm down and near to the body. Bring the heels and toes together and keep them loose. Make the whole body straight and look towards ceiling. Breathe normally.

Steps
Exhale and Inhale deeply while raising your both legs and spine until they make angle of 90 degree with the floor. Attain the Uttanpadasana posture.
Exhaling raise the waist and push the legs backward over the head. Support the waist with both the hands, using your hands get the legs, waist and back in one straight line and stabilize them in this position, stretch the toes towards ceiling. Keep your spine, legs straight and sight on the toes.
when the body has been raised to a maximum point stay there and breathe slowly and deeply with the abdomen and concentrate on the thyroid gland. Chin is placed in the Jugular notch forming a tie which is called 'Jalandhar Bandha'. Stay in this position for ten to fifteen seconds during first week. Gradually develop the time to one minute in a month.
To come out of this posture, Inhale and exhaling, slightly lower the legs towards head and maintaining the balance of the body remove the hands supporting the waist back to the normal position. Inhaling , place the waist on the ground keeping legs straight as in the Uttanpadasana. Exhaling, bring legs back to the normal position without jerk or speed.
 

Benefits
It stimulates the thyroid gland to work at peak efficiency. It's the thyroid gland which is mainly responsible for your correct weight and youthful appearance (it controls metabolism). To some extent pituitary gland also get stimulated thus helps improve their function.
The shoulder stand also regulates the sex glands.
It vitalizes the nerves, purifies the blood. This helps improve the blood circulation as impure blood easily reaches the heart
It gives a healthy stretch to the neck muscles.
It is beneficial for people suffering from poor circulation, constipation, indigestion, asthma and reduced virility.
It also helps cure varicose veins and haemorrhoids. This pose is especially recommended for women after childbirth and for those suffering from painful menstruation, other female disorders, and seminal weakness.
For the asthematics, Sarvanga asana has medical value. It exercises all the bronchioles and totality of lungs.The whole respiratory system is invigorated and strengthens.


Time
Retain this position for fifteen seconds to six minutes, adding fifteen seconds per week.
Sarvangasana is done only once. It is not repeatable asana.
 

Caution
Do not try this exercise if you are suffering from organic disorders of the thyroid gland. Be very cautious if you are suffering from chronic nasal catarrh. People suffering from headache, brain diseases, blood pressure, heart related ailments etc. should consult their yoga expert before doing this Asana.
 

Note

It is recomended to practice the Matsya Asana after releasing the Sarvangasana. During Sarvangasana the head, the neck, the shoulders becomes passive and the lower area of the body become active. In order to create a balance Sarvangasana is followed by Matsya Asana