Sleep stages and circadian rhythms

Sleep stages and circadian rhythms are a fascinating subject and very much related to the innerdance process. 
To get a basic understanding of sleep stages start here: https://youtu.be/fNlp0UMqUtM
And if you want to dive deeper and explore the states of consciousness this is a very good playlist : https://youtube.com/playlist?list=PLbKSbFnKYVY3_Pav6PyqZg5tgaeCUCAa7