The NormalBreathing.com website has several pages and learning Modules devoted to sleep (see Resources below). However, you can find more details related to sleep and how to get higher results for the morning body O2 test in this ebook "Sleep Better and Less Naturally".
This book summarizes Dr. Artour Rakhimov's clinical experience with thousands of students who learned the Buteyko breathing method and other techniques. It provides a systematic and detailed approach that explains the role of sleep in health and how to improve sleep quality naturally.