I did it in my late teenage years, alongside many 24-48 hour religiously-motivated fasts. (In this case, they were water-only.) 24 hours isn't two bad. The sec

I did it in my late teenage years, alongside many 24-48 hour religiously-motivated fasts. (In this case, they were water-only.) 24 hours isn't two bad. The second 24 hours is awful. That's when the headaches come. The third 24 hours your body starts stabilizing and it's pretty doable from there. Friends who went longer said it gets bad again in the second week.