My friend, who fasts regularly (1-2 times a year) for 7 days at a time, drinking water, tea, or coconut water only, said that she has more dreams than usual during fasting. When I tried the same nutritional regimen, I too, unexpectedly experienced more dreams than usual during the latter part of the 7 days, and the dreams are more unusual as well.
NOTE: I started the fasting as a skeptic, only for weight loss, NOT believing that there is a link, and certainly NOT with (what I think is) a superstitious, albeit religious, motivation & expectation of my friend (see below).
Although my ultimate question is to evaluate her claim that religiously motivated fasting results in God's speaking to her more (to give her either a prophetic blessing or a warning for spiritual warfare by opening her vision to the unseen realm), I purposely restrict this question to scientific linking of water fasting to more dreaming without regards to the interpretation of the dreams. But if one of the study variable is "motivation for fasting", the answer would be even better.
A quick Google search gives me a short FAQ answer Why do I get strange dreams while fasting?.
Lots of people report changes in sleep habits when they start fasting. Most commonly, fasters report sleeping more deeply and having more vivid dreams at night. The effects of fasting on sleep quality and duration have only been studied in two trials to date.
It points to a pilot study of 7 day fasting quite close in regimen to water fasting, indicating increased REM sleep, which can be downloaded from JSTOR. I'm looking for other studies.