My second last fast was 6.5 days and my most recent 11 days (just finished a few days ago).
Sleep comes and goes… I agree with what others say, accept that there will not be a lot of sleep, relax any stress you feel from thoughts that you must have sleep at certain times for certain lengths, rest when you can. Take it easy. Night air is wonderful. Eventually my open window felt like I couldn’t get enough of it so I slept outside. Laying outside looking at the stars listening to some gentle music. Tops.
I used to not be able to sleep even on day 3 but now Before I want to sleep I take a relaxed walk at night through the neighborhood while doing conscious breathing through the nose only, with the mouth closed and the tongue on the roof of the mouth. I do this for 1 to 2 hours every dry fast night before bed and I get a full nights sleep. Although around the 5 or 6 day mark sleep can just become difficult no matter what you do. May just need to be embraced