Sadly I’m the opposite. Multi-day fasts for me take so much self-discipline and motivation that I’m left with very little self-discipline to do even basic things like a solid day of work. It is a complete mystery to me how depriving oneself of food seems to have the opposite effect on some people. Lucky bastards!
Mental clarity, motivation, and productivity all go through the roof for me. Water fasting with black coffee feels absolutely incredible for me up to about six days. At six or seven days in, I start to feel like complete crap so I typically only go for around five days at the longest.