Source: http://nymag.com/scienceofus/2014/08/sle...

What it comes down to, she thinks, is the connection between sleep and temperature. Sleep researchers know that right before you fall asleep, your body temperature starts to drop; in the deepest stages of sleep, your body is at its coolest, about one or two degrees below normal. Some scientists believe cooler temperatures cause sleepiness, and although the pre-slumber cooling process happens naturally, there are a few things you can do to help it along, like taking a warm bath right before bed, for example. When you leave the tub, your body temperature rapidly cools, triggering that sleepy feeling. A warm beverage works the same way.