It's a great feeling to be reading and all of a sudden you notice it got dark outside and you have nto eaten anything all day, because you were just so involved int eh story....
It's usually the headache from the low blood sugar that makes you look up and notice it is dark out.
@Surfpirate That sound abotu right... only I don't usually get headaches for that reason, but I do notice my brain and general energy has slowed down at that point.