PreciousBooBoo, you do not need advice from Dr Mosley about dehydration.
No where in his book does it say you should not drink water, far from it . On any form of diet you need to stay hydrated, that means drinking sufficient water for your bodies needs. Going without water is to say the least foolish. Drink water normally during your day and during your “fast days” and your body will be ok. If you have any doubts just google Hydration and you will get the info you need.
I would also suggest you see your Doctor. He is more qualified to give you advice regarding your symptoms and hydration than any one on this board.
Good luck.

PreciousBooBoo

If you think you are still dehydrated drink more ?

The best guide to knowing whether you are truly dehydrated is the colour of your urine! Brown is bad – defo drink loads, any water in tea, squash, melon whatever counts, but just plain water is zero calories so doesn’t eat into your 500 cals. Light pale clear yellow is good, you are fine. When they talk about losing water weight, this is because one fat cell, is not only fat, but also about 3/4 water and 1/4 fat. So when you lose body fat, you are also losing water, but this is from what was tied up in your fat calls rather than free in you body. Hope this helps. Also v good for your kidneys to drink plenty.

PreciousBooBoo, I want to assure you I had the EXACT same situation, when I started the diet at the end of June. The first fast night I slept so-so, the second feast night I slept just a few hours, and by the third feast night, I slept, no exaggeration, 1-1/2 restless hours. I normally sleep 9 peaceful hours a night. Even with the mass sleep deficit, the next few nights continued with very limited hours of sleep. I took magnesium, I got up several times to drink water thinking I was dehydrated, I had no caffeine during day, etc. I thought I might have to go off the diet. The good news is that the insomnia issues lessened, and within two weeks, I do not have insomnia any more. I will say I sleep less… I go to bed later and wake up earlier, but I consider that a positive thing, because I’m not tired during the day. So push through, and this insomnia issue will probably pass for you, too.