|MadSci Network: Medicine|
The answer to this question is most easily answered by harking back to mans' caveman past.
In such times, people never knew when or where the next meal would be. When they hadn't eaten in a while, eg in winter when some of their prey were hibenating and fruit and vegetable food sources were low due to winter, our predecessors bodies would start to conserve as much energy as possible to help them get through the fast. The body would slow down metabolism and reduce energy use by slowing non-essential bodily processes. Hence the lethargy and tiredness.
In today's culture sometimes it is inconvienent to eat regularly, or the desire to lose weight can force people to skip meals. When this happens, glucose and glycogen levels start to deplete and the body starts to prepare itself for a fast by slowing metabolism, making the indivdual lethargic and tired (hypoglycemia).
NB: For this reason, it is NOT a good idea to diet by fasting. As the body slows down, less energy is burned so not as much fat will be lost as when eating sensibly and exercising. Also when the faster resumes eating, the body sucks up any energy it can and converts it to fat in anticipation of another fast.
Try the links in the MadSci Library for more information on Medicine.