Is it bad to eat at night and if it is, why?

Yes and no: Late night eating will lead to weight gain as your bodi's metabolism slows and the food you eat essentially gets stored as fat. Also depends on what you are eating. Not going to harm you thogh.

Think Sumo Wrestlers: Sumo wrestlers want to put on weight, and so they eat high carb/high fat meals at lunchtime and take a nap. Then they do the same thing at dinnertime and go to bed to sleep. It is best to take in your daily calories (more protein) in the morning, less at lunch, and less before bed. Use the energy you eat during the times of day you are going to be active - not when you will tend to store it away.