The NY Times has a great article on sleep and exercise. Ever wonder why you have more energy after an easy jog but after a hard run you are beat? Turns out it has to do with your immune system:

One possibility, Dr. Chediak said, is that cytokines — hormones that signal the immune system — are making these athletes sleep so much.

Exercise, Dr. Chediak said, prompts muscles to release two cytokines, interleukin 6 and tumor necrosis factor alpha, that make people drowsy and prolong the time they remain sleeping. In fact, those cytokines also are released when people have a cold or infection, which is why people sleep so much when they are ill.

Link.