I'll give you a brief primer. Leptin is the hormone that signals our DA receptors in our brain (brain in general) that we have enough fat to survive. Now, leptin can be held at an alright level if you maintain some bf, and maximize health through various factors. But, for those trying to reach say single digit, leptin is your new enemy. Your body doesn't exactly want you to get to single BF unless you're already there naturally, or can get there relatively easy without much effort.
Now, with dieting, like most due even on primal, leptin levels drop to about 50 percent after just a week... Losing sleep will also result in a leptin crash the next day which you will notice through increased hunger.
Leptin virtually shuts hunger off through OB-Rs --> OB-Rl-->DA1/DA2-->DA--> Neuropeptide Y down and Corticotropin Releasing Hormone normalized
What that does is normalize (usually lower) cortisol levels, raise testosterone, and raise Thyroid stimulating hormone, and thus T3/T4.
So after a re-feed generally test. is up, thyroid is up, leptin is up, and you're ready to burn some bf with less hunger...at-least for a few days until the next re-feed.
Also normal cortisol is necessary for proper thyroid function because cortisol helps cells receive thyroid hormone. Thyroid hormone can than regulate your sex hormones. It's all interconnected.
This is when you're trying to get to single digit though. I can't exactly vouch for whether it's healthier or less healthy, and anyone who says they can would be going based off opinion. The key to doing it is to do it right, meaning:
high carbs (roughly 50 grams every 2 hours)
low-fat (50 grams or less to maximize) on re-feed day
moderate protein (not too much because this will zap hunger)
exercise before re-feed, not during (during screws with the hormonal fix-up)
I'd advise everyone check out Lyle McDonald's books for more info. They have a lot of great info and he goes into great detail on all of these underground methods.