It’s because you are over stimulated by where you’re placing your attention.  You’re measuring where you are now relative to another place and time.  Making your body lethargic is your soul’s way of telling you to slow down and turn your focus within.  You’re unfolding more now in a spiritual sense than in a material sense. A journey within will be very fruitful now.  Sleep as much as you need to.  My experience is, when you’re on the Path, you begin going through a series of sleeper’s classes.  If you’re awakened, hit the meditation mat. As you follow your breath, you’re giving your unconscious mind time and space to integrate what it’s just learned. You can be half asleep, when you doze, your head falling will awaken you to follow your breath again. Give them 20 minutes and go back to bed. Then watch everything in your life begin to all come together.