As I believe, Astral Projection (Dreamwalking) ties in with dreams.
Dreams only truly occur when we sleep. True, we have "daydreams," but these are really nothing more than thought out fantasies, and don't carry the weight of dreams. When we sleep, our body all but shuts down. Our heart-rate and breathing slow down, our body temperature cools down, and our muscles relax. But is this what leaves us feeling rejuvenated after waking?
What makes us tired? Physical labor can wear us out, and remaining awake and active can do the same. We become sleepy, however, when our Spirits (the source of our energy and life,) wear out our bodies. When we sleep, as opposed to when we nap or rest, our Spirits partially leave our bodies. This is what causes our body to "shut down" to the bare necessities. During this time, we dream.
While our Spirit has left our body (not fully, leaving enough to keep us alive and anchor the Spirit to the Mortal Plane still,) it is "unhindered" by the mortality of our body. This allows us - as Spirit - to essentially "vent" and wind our Spirit down. We wake refreshed because we have a rested Spirit, though as we progress through the day and draw upon its energy the Spirit wears down our bodies. This is the source of exhaustion and aging.
Dreams are bridges to the realms of the Divine. Often we hear of the Gods visiting man in his dreams, or inducing a "deep sleep" so that mankind can be communed with. This is because to commune clearly with us, our Spirit needs to be unhindered by the noise of the Mortal world. While dreams are the primary source of connection to the Divine, this can also be attained through forms of meditation and ritual practice. Certain frequencies (easiest found in music,) help our Spirit to bridge the divide, making a brief touch to the Divine. This is why some songs or musical scores are "moving," or inspiring.
Through dreams, we walk where mortal feet cannot go, and can even learn to exercise our creative abilities by controlling our Spirits and influencing the realm around us (Dreamwalking). As we are in the Spirit realms, during our sleep we are more receptive and open to Spirits, whether they be the Divine, elementals, angels or daemons. The more darker Spirits influence us in the manner of nightmares. However when we wander the Spirit realms, we take the risk of separating from our bodies. Of course, if this happens, we die.
Often when we wake from a dream, we can't remember it. Such events are due to the veiled nature of the Divine realm. Many religions teach that "such matters" are not for humans to know, and this is true. Often we receive startling revelations or realizations while dreaming, and though our Spirit may think to itself 'I will remember this when I wake!' it is almost never the case. Other dreams, such as when the Gods speak to us, or when we receive a revelation that is meant to change us mortally, we do remember them. Such dreams often stick with us for years, and are hard to "get rid" of.