Deep sleep is a state full of peace. The troubles of the waking world, starting from your mordgage to the annoying neighbor are not there in deep sleep. So it's restfulness, a peacefulness. In verantay speak about anan the maya kosha,. or the blissful, sheat that is the experience of deep sleep. It isn't sachi dananda, the ultimate bliss itself, the brahman itself. We just don't know it.