Our evolutionary ancestors regularly fasted, though no doubt involuntarily, so we have the ability to survive a short-duration fast with no harmful effects. Many ancient philosophers -- Socrates, Epictetus, Diogenes, et. al. -- claimed a short fast cleared their minds and bodies.
About once per month I do a 48-hour fast, ingesting only water. If I am feeling fine, I _MAY_ extend it for another 24 hours. It does indeed seem to perk me up. My thick head seems to be not-quite-so-thick and a five-mile walk/light jog is easily done with a light euphoria after sitting down (not quite the same as the famous "runner's high"). There is no apparent change in my usual heart rate either during or after the exercise and my blood pressure is the same. After my first light meal -- usually a small serving of beef or chicken and a small salad -- I feel seriously fine.
Does anyone else try fasting of any duration? What are your results? Either post it here or email me at firstname.lastname@example.org. This is not any sort of study, I am just curious.