Why is eating after 23:00 unhealthy?

The rule is actually that you are supposed to stop eating 3 hours before you go to bed.  I guess it takes 3 hours for your food to digest and giving yourself that time helps with managing your weight or metabolism, and also helps you get a better night's sleep. 

See these sites for reference:

http://coaches.aol.com/diet/jorge-cruise/rev-your-metabolism

http://coaches.aol.com/wellness/michael-breus/tips-for-sleep

A careful reading of the annotation to the question reveals the following :

i) it is a personal unaviodable situation. ii) whether late dinner does harm to you is not clear.iii) the body is used to this kind of living. Now, the third fact almost answers your doubts. A dietician may prescribe, but it is you who have to observe what is happening.To eat is human; to digest is divine! Add prayer to the menu. It works. When a body requires glucose administration at zero hour it is given immediately, is it not? Take liquids.

It depends on what extent is your night eating goes. It could be an actual eating disorder but otherwise most studies are saying the worst effect is heartburn.  Just in case you might want to read about night eating disorder