From a collection of articles I read, the normal setting is really for heavily soiled dishes, light wash should cover you for most washes.  The difference is in the length of the cycle and number of tub fills/drains (a typical light wash may be about 80 mins, 3 tub fills, Normal being 1 1/2-2 hrs, 4-5 tub fills).  In most cases you should be good with the light wash, so give it a try today and save on water, energy and money!