Do you want to know the first practice for mastering sleep? Not just the relaxations you do. Not going into sleep direct from wakefulness, and not going into sleep through that mental reverie ---all kind of imaginations that go on. Here, let me give you a universal principle about the practice of yoga. When you have done a leftward movement, and you want to start a rightward movement, what do you do in between? You first come to the center! You don't change the direction without first coming to the center. You change nothing without first coming to the center! So, from wakeful state to sleep state you come to the center. You enter the state of meditation. From there you go into the sleep state. And, upon waking up, you do not go from the sleep state to the wakeful state. Because you have been observing the fact that that part of your mind has been asleep, so, as soon as that part has rested sufficiently and you have observed that this part of the mind has rested sufficiently from the sleep state, you go into meditation state. And, from there, you come to the wakeful state. So even if you have had one hour of sleep and you had missed two nights of sleep before, out of that one hour of sleep, you will not wake up groggy, but you will wake up alert.