Why do I get reflux late at night?

Reflux of acid in the oesophagus is caused either by excessive acid in the stomach or decreased emptying of the stomach,

Both of those things happen after you have eaten. People who eat late at night are often people who are then going to lie down and watch TV, or to go to bed, and when you lie down you now have gravity assisting the contents of the stomach going up into the oesophagus instead of forward into your small intestine. People who are eating late at night should either not lie down for a couple of hours after their meal, or should eat smaller meals. It is also advisable not to drink with meals, especially cold drinks or alcohol. Propping the head of the bed up by about six inches may be useful.