I haven’t had the headache problem usually running 16:8. Feeling hungry about six hours in is a years-long observation.
With caffeine withdrawal the headaches subside after a maximum of two weeks. I would power through it to see if the fasting headaches go away. Best of luck.
I don’t often have headaches during fasting but when I do it is usually because I had something sugary near the end of my eating window.If this is not your case, you could try a pinch of salt in your water, or directly on the tongue, that usually helps with the headache.
Dr Jason Fung said that is likely caused by salt loss and taking in a little salt during your fast in the beginning will help. You can mix a pinch of salt in a glass of water or have a little bone broth.