Atrial fibrillation. Fast and irregular heartbeats could imply atrial fibrillation. This is an arrythmia wherein their is an uncoupling of the electrical synchronization between the upper cardiac chambers (atria) and lower chambers (ventricles). This leads to a fast and irregularly irregular (irregular cadence). This can result in light headedness and potentially in clot formation and stroke risk.
Fasting. Likely no, fasting will not help irregular heart beats. However, staying well hydrated may help and avoiding certain things like alcohol, caffeine and tobacco use will help with decreasing irregular heart beats.