Savasana is a pose of conscious relaxation. Although many people fall asleep while practicing savasana, that is not the aim of this pose. In Light on Pranayama, B.K.S Iyengar says, “Some call it the Eternal Now, beyond space and time. Others call it the soul becoming one with the Creator. This can be experienced in perfect savasana when the body is at rest as in deep sleep, the senses as in a dream but the intellect alert and aware.”
Savasana is a restful pose, and the outcome of it is recuperation and rejuvenation. Sleep, however is not always restful, and it's possible to even wake from sleep and not feel rejuvenated. Why is this?
Studies have been done that measure the brainwave activity of those people who practice conscious relaxation and found that alpha waves (slow brain waves) increase in intensity and frequency during the practice. Interestingly, in similar studies done on sleep, these alpha waves are not commonly found during sleep, or aren't an overarching and ever-present component of sleep. During sleep, the brainwave pattern changes depending on the depth and stage of sleep. Further, sleep can be un-restful due to insomnia, vivid and disturbing dreams, frequent waking etcetera, etcetera.
With meditation and conscious relaxation however, the brain kind of downshifts to an idle, the alpha state, where sensory input is less, awareness is internal, and the brain is in a state of non-arousal. Alpha brainwaves are the resting state of the brain, and aid in contemplation, reflection, mental coordination, calmness, mind/body integration, learning and creativity.
Practicing meditation and savasana nurture the alpha state. With regular practice, the practitioner to more easily and readily achieves the alpha state. Studies have also shown that practices that employ alpha brainwave activity may have a regulatory role on sleep, meaning that people who practice conscious relaxation and meditation, sleep more deeply and more restfully when it's time to sleep.
As I've said before, savasana isn't about sleeping...but so often, people do fall asleep in savasana. If you aren't getting enough sleep, or enough restful sleep, falling asleep in savasana would make sense. My yoga teacher used to say, "if you fall asleep, you need the sleep." So true.
Sleep obviously has it's benefits: physical recovery, cognitive organization, improved ability to concentrate and learn, memory, and mood regulation to name a few. But, as the buddhist saying goes: "If you walk, just walk. If you sit, just sit; but whatever you do, don't wobble" - when it's time to sleep, sleep well and when it's time to relax, relax well.
As I've mentioned before, savasana is a practice. If you find you are falling asleep, try shortening the time you are in savasana. Decrease it from 10 minutes to 5 minutes to work on developing the ability of the mind to stay focused and alert without falling asleep.
The more you can train your body to be still, and your mind to downshift and idle in a relaxed state, the more easily and often you will stimulate the relaxation response, thereby making it more accessible. Overtime, your ability to stay consciously relaxed in savasana will improve, and perhaps your ability to sleep deeply at bedtime will improve as well.