You can get tired, cranky and angry easily and fast. It’s much worse if your sleep has been bad recently. You can make really poor decisions and offend people or do stupid things you’ll regret because of non-existent willpower. You can also faint which is dangerous even if you fall down relatively safely, never mind if you crack your head open or if you’re in the middle of doing something even moderately dangerous.
Start fasting by skipping breakfast and work your way up to skipping lunch and then doing a full day fast if you’re going to do it and always stay hydrated. If you’re doing a long fast make sure you’re getting salt too, even if it’s not hot enough that you’re sweating much. Take it easy and go slowly like with starting a new exercise regimen.