Then said his disciples, Lord, if he sleep,.... Soundly, quietly, and comfortably, and takes rest in it:he shall do well; or "be saved" from the disease; he will be delivered from it; he will recover out of it; it is a sign the distemper is leaving him, and he is growing better, and will be restored to his health again: the Ethiopic version renders it by many words, "he will be well", and "will awake", and "will live".
Sound sleep is a sign of health.