I noticed a similar tendency in my own behavior. There are two things that I've done about it. First, around an hour before bedtime, all the electronics go off. Pull out a book of whatever you like to read for recreation. Force yourself to start reading, but don't force yourself to keep reading, because then it will feel like a chore. The deal is that you can read as long as you'd like, but when you put the book down, the light needs to go off too. Second, almost everyone has some kind of creative endeavor that they can pursue on their computer. In my case, there are a small number of programming projects that I've started. Writing software requires long periods of concentration, and if you're working on something interesting, then you'll have more incentive to stick to the project.
If you're unhappy with your level of concentration, then find something you enjoy concentrating on. Then when you're obligated to do something tedious, you at least have the attention span to properly apply yourself to the task.