Hypoglycemia is a condition where blood sugar drops below normal levels (strictly defined as a blood glucose level below 70 mg/dL). People with this condition often have blood sugar levels that fluctuate from above normal to below normal.

Some of the symptoms of hypoglycemia:
The most noticable of these is fatigue. Someone who is hypoglycemic can go from being alert to completely lethargic very quickly if they do not eat properly.

Some of the causes of hypoglycemia:
In some cases, hypoglycemia is treated with glucagon, but for the most part, it is treated with a very strict diet. A snack should be available at all times, to help if the person becomes hypoglycemic. Someone who is hypoglycemic needs to eat frequently, eating a couple small meals in place of one large meal.

My best friend is hypoglycemic, so I have researched this from time to time, and have witnessed some of the symptoms firsthand. Further research can be done on the web. A good place to start is http://home.earthlink.net/~dougwheel/hypo/index.html