I've found that I can manage a day without taking in any calories, if I'm willing to feel really tired and have a moderate headache by ~1pm, and be horribly brain-fogged and useless by ~2pm. I've got a job and kids to take care of—I can't afford to have days like that very often. The hunger's not a problem—it's that I feel rather ill by mid-afternoon.
FWIW I rarely eat breakfast, so I guess I'm usually doing the (lame, IMO) "16/8" fasting pattern, though I'm skeptical that has any benefit.
The reason you feel sick is because you're body isn't used to the shift from carbohydrates for energy and ketones as energy (body fat). You get brain fog because only fat in the form of ketones can cross the blood brain barrier. So adaptation to ketones are vital. You need to work into it. No different from exercise, or climate adaptation.
Having cold showers shifts your body into using ketones more and more.
Regarding not eating breakfast, watch these videos from FoundMyFitness:
Dr. Satchin Panda on Time-Restricted Feeding and Its Effects on Obesity, Muscle Mass and Heart Health
Time restricted eating, AKA your 16/8, is VERY important. Even if you don't immediately notice changes. Those changes could begin to occur 2 to 8 years later. Depending on adherence. Also based on Dr. Ruth Patterson's advice, skipping breakfast leads to eating late into the night, which is not good for your health generally.
FWIW I rarely eat breakfast, so I guess I'm usually doing the (lame, IMO) "16/8" fasting pattern, though I'm skeptical that has any benefit.