Savasana is considered a pratyahara practice. Pratyahara means sense withdrawal. Think of this as training for meditation. When you are in savasana, you are practicing letting go of your attachment to the body, the sense organs, and the outside world. You are encouraging your mind to go inward. Savasana is the opportunity to relax your mind in a quiet body so that you can assimilate your practice; therefore, it is offered at the end of a session. Savasana is a state of conscious surrender - a time to rest, yet not a time for sleep. Deep relaxation and an elimination of stress/tension from the body are meant to put the mind in a place where it can let go, too.