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In my experience your body "learns" when you normally eat and makes you hungry/cranky at that time. You can re-train it, and the hunger/crankiness disappears.

For example when I started doing IM (no eating from 7 pm - 11 am) I'd get cranky around breakfast time for the first few days, and then it went, and my most productive time is now 8 am - 10:30 am.

Same thing with OMAD or regular fasting, you can retrain your body into what is normal, and you do gain better focus. You do crash fairly hard around your new eating window though.

Yes same for me. Fasting was incompatible with concentrating.

That means you are pushing your limits. Don't start with a full day fasting. Start slow by doing some form of intermittent fasting or time restricted feeding. I would say go for having early dinner and a late breakfast to begin with. Once you are adapted to log hours of not eating, skip one of the meals in the day, depends on your situation but the easiest one to start with is skipping breakfast. Then you can slowly work your way to full day fasting.

I take exogenous ketones to ease into the fast.