When we overwork, the line between work and personal time blurs such that they eventually become the same. So you don't feel guilty about slacking off.

Long hours in an office setting promote procrastination. Why do something now when you are going to be here at 8PM anyway and will need something to occupy your time?

Once you stop procrastinating, because you know you can't be at work late, then you build up work inertia. This is probably where all the efficiency gains are.