Ultimately the choice to lie down or rest for savasana is up to the individual and one must pay attention to the effect of doing or not doing it and see how the individual body responds in different circumstances – remember every moment and every practice is different so the aim of the practitioner is to learn to respond to what is needed at each moment.

Here is a post by our friend and yogic blogger Anthony Grim Hall – Anthony has included the following extracts from our book the ‘Applied Anatomy and Physiology of Yoga’ by Bianca Machliss and Simon Borg-Olivier have written about Savasana:
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“The supine yoga relaxation (savasana) for 5-15 minutes at the end of each yoga practice is important for many people. Recent studies [Bera et al., 1998] have revealed that the effects of physical stress were reversed in signifcantly shorter time in savasana, compared to the resting posture in a chair and a supine posture. In savasana the muscles can be fully relaxed if they have been stimulated by either stretch or activation during the practice. However, if the nervous system was over-stimulated during the practice then relaxation will still be dificult. The brain can relax if it has been engaged throughout the practice in the process of either focusing on a particular type of breathing, or feeling the sensations of intelligently organised stretching and activation. If the brain was not engaged in the functioning of the body in the yoga exercises then it will be less able to relax and more likely to become either restless or sleepy”. p.66
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“While the aim of your yoga practice should be to remain as relaxed as possible during the practice, for most people at the end of each yoga practice it is important to take at least 5 – 15 minutes of relaxed sitting or supine relaxation (Savasana) with observation of the breath to further relax the body and brain. This relaxation is most successful if people have succeeded in getting out of their brain and into their body with physical practice of asana (static postures) and vinyasa (dynamic exercises) beforehand”. p.265
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“New practitioners of Patabbhi Jois’ astanga vinyasa yoga tend to hyperventilate during the entire physical part of their practice with deep relatively fast breathing. Although this type of hyperventilation confers several benefits – trunk strength, flexibility, cardiovascular fitness and mental focus – it can elicit many adverse reactions including emotional instability, excessive hunger and others listed above. These can usually be countered by a subsequent period of supine relaxation (savasana) of ten to thirty minutes in which natural hypoventilation (minimal breathing) is performed”. p.364

This is a rolling post from Anthony Grim Hall 2013, updated in 2017 – see this link for the full post
http://grimmly2007.blogspot.com.au/2017/01/why-take-savasanarest-at-all.html

“…After completing their yoga practice consisting of asana and pranayama, the yoga practitioner must rest for fifteen minutes keeping the body on the floor before coming outside. If you come outdoors soon after completing yogabhyasa, the breeze will enter the body through the minute pores on the skin and cause many kinds of disease. Therefore, one should stay inside until the sweat subsides, rub the body nicely and sit contentedly and rest for a short period.
Krishnamacharya Yoga Makaranda (Mysore 1934) p34