@Inna You’ve asked this same question in another general topic. I suggest you start a new topic to discuss only this issue. This particular topic (‘Off Topic Forums’) is not the place since few continue to view old topics like this. I’m sure if you started a new topic, you would get lots more responses and maybe even some useful/helpful advise about it.
Just a quick comment, though. If you get light-headed after 20 hours of fasting, something’s not right. Maybe you are referring to pre-keto experiences when you were a sugar-burner? With your brain running on glucose, you can get light-headed not eating, although even then there is probably something amiss, for example low blood pressure, rather than simply a lack of glucose. We evolved quite effectively to maintain brain activity regardless of intermittant energy intake. If you are consistently in ketosis, your brain is running predominantly on ketones. As long as you have body fat available to convert to ketones, your brain will continue to function normally during a fast. Also, prior experience as a sugar-burner may prompt you to fear severe hunger from fasting. But that doesn’t happen while you’re in ketosis. ‘Keto hunger’ is a very different experience than ‘carb hunger’.