That is why people who go crazy are called lunatics. The word itself says lunatics.
And that is why our ancestors have said to fast on Ekadashi (eleventh lunar day of the Hindu calendar). Why have they said this is because when the stomach is kept empty the toxins in the body get cleared. Fasting has a purifying effect on the body and it cleanses all the undigested food, toxins, etc.
So if you fast three days before Full Moon then on Full Moon day you will not have any illnesses – this is the belief.
It is not necessary that you must fast on every Ekadashi but it is good if you fast for at least two to three days in a year.