My experience was that the heart racing, dizziness & anxiety goes away after a few experiences with IF, maybe 2–3 weeks worth. What I notice since then is much more like absolute calm. Your body learns to relax and know that food is coming at some point, but we have plenty of reserves to make it through.
IF is a kind of induced stress to your system, sure, but it’s controlled, so you aren’t actually worried that you’ll never eat again.
What really helped me, and what started all of this off for me, was the science and studies shown in Eat Stop Eat. I bought that book in 2009 and I would buy it again for 100x the price. Trusting the science in that book is what helped me relax and know that I was okay. I would recommend you read that book from front to back. www.eatstopeat.com
Fasting for entire days is healthy and was even a way to help your body heal from different ailments in centuries past.
To answer your calorie question, my target is around 2200–2500 calories per day, but fasting, if I only had 1 meal, it might be more like 600–1000.
To note on your sleep issue, getting the right amount of sleep will make everything else in your life better. If I didn’t get enough sleep, it’s more difficult to do things that require my willpower, so keep adding minutes to your sleep schedule, get the room nice and dark and get your phone away from your bed. If I hit 7.5 hrs of sleep, I wake up a totally different person than if I only got 6.5.
