Because of my current lifestyle, practising The Phase at night is challenging. Generally I sleep only 6 hours per night and if I disturb that pattern I'm left feeling pretty disoriented during the following day. However, I am fortunate in that I am able to take an nap during the day and I've found that this is a good time for me to experiment with 'The Phase'.
When I first started practising, as is my nature, I was so keen and enthusiastic that, despite the warnings in the book to the contrary, I would try and awaken during sleep whenever I could, but I soon stopped this in exchange for something more sensible once I became sleep deprived.
The nap is the sensible option for me. The main thing I have found is not to try to cut short my main bulk of sleep at night. It's also suggested in the book that you only try this for a few times a week but of course I won't be doing that either! I learn best through my own experience.