The Science of Fasting

Okay, I found and read this thread over the weekend. I watched the documentary posted by Expat in the OP. Fascinating and almost unbelievable.

I intend to do more research and re-read the thread because of the many good posts.

I'd like to give this a go, but I don't want to do it unsafely and I haven't done the homework. Because of lent I'm attempting a fast for Ash Wednesday (last meal was last night at about 19:00). What are the rules to fasting - just water? Any nuts or broth, etc.?

I'm hoping to go at least 36 hours.

And does someone have a link to the 8-hour eating window that was mentioned earlier?

I'm sure I'll be back with more questions, later - if I don't pass out. Thanks in advance.