My understanding is that sleep does not have to interfere. Intention can help a lot. Tell yourself before you begin, "If I fall asleep, the momentum of the energy will continue to go around and cycle through the small universe. This will be healing to me in every way and I when I awake, I will feel rested and refreshed.".
From my experience in both this and other qigong systems, once you get the flow going, it continues even without your conscious mind for some time.
And remember: good, better, best. It is still good to do it now matter how or when. And you will get benefit.
Hope that helps.
