Most people think (like I did) that because they think a sugar crash is the same thing as hunger. Hunger doesn't feel anywhere near as bad. Once you're in ketosis, which is the 72 hour mark (give or take) of water-only fasting you actually feel great. And if you do a keto or paleo diet you won't feel bad in the first place.

My longest fast was 9.5 days and I felt great the whole time and lost a ton of weight amongst various other benefits.

Remember, humans used to fast all the time and it was just normal because either food was unavailable (there didn't used to be unlimited pop tarts for everyone) or for spiritual reasons. Both what we eat and when we eat were all invented in the last few hundred years for the most part (chicken was about 100 years ago, breakfast was 2-300 years ago depending when you count it). Most of these inventions have been a bad idea.

Come join us on /r/fasting and /r/keto on reddit, or read Dr. Fung's books on obesity and on fasting, or Gary Taubes various books, or Dr. Wahls books, or Prof. Seyfried's textbook.