From Greek: endo (ενδο) = within and Morpheus (Μορφέας) God of dreams.
So we're dealing with the dreams within us, but actually it relates more to a pharmacological activity in our body that is triggered by neurotransmitters.
Because we writer's shouldn't forget that exercise and the outside world is just as important. We shouldn't underestimate the power of endorphins. Whenever I come home from yoga or a good workout or maybe just a swim, I feel much more productive. It's like a rush, especially when I had some sugar. I would call this a sugar-endorphin rush.