This is mainly because eating a big meal raises the glucose levels in our blood. High glucose levels then have the effect of inhibiting, or even switching off, the brain cells called orexin neurons. These are the neurons we need to stay awake - they slow right down at night, and if anything goes wrong with them it can need to narcolepsy, where people just fall asleep all the time.

So the more you eat, the sleepier you get. In the same way, if you are hungry it's often harder to sleep, as the orexin neurons become more active the less glucose there is in the bloodstream. It's thought all this may make evolutionary sense: if you haven't got any food you need to be alert so you can get some, and on the other hand if you've eaten a lot, it makes sense to slow down or cease activity so you don't burn off the fuel too quickly.