I like to think of our internal clock to be like a yin-yang symbol: there is a part in the afternoon where we should sleep and in the early morning where we should wake up briefly. The early morning part is likely there for us to get up and stoke the fire or stand watch. In the modern day, it's really useful to wake up at 3am and meditate before going back to sleep. Not only do you feel more calm and refreshed for the day, but you'll also have intense, vivid dreams. I often get insight from these visions; there's something mystical about them!