I'm a big fan of Equate Pain Reliever PMs (that's basically a cheaper equivalent to Tylenol PM; it helps me sleep), but on a chemical scale, how do these drugs work? Is it that they require so much energy to break down and digest that the loss of energy is what leaves you sleepy? How do they work?

Sleepiness is caused by the antihistamine in the product (much like you would get tired after taking Benedryl... a name brand allergy medication/antihistamine).

 

Much more info here:

http://www.drugs.com/mtm/tylenol-pm.html