Most people when fasting are in fact weak because they are detoxing. Switching into very light, digestively simple eating after a long fast will provide the majority of people with far greater energy, primarily because it greatly diminishes the level of this detox.
The truism that heavy foods put us to sleep after a long fast is not to say that light foods will do the same. Even whole fruits in quantities too large can make us feel sluggish and bogged down from the digestive over-burden that our fasted body was not ready for... and can make us feel even weaker than we did during the long fast. The importance of gradually easing out of a long fast can not be understated. An avocado to break a 40 day fast is extremely aggressive as compared to say 1/8 cup of well masticated fresh squeezed orange juice and the difference in how each makes us feel will in most cases be monumental.