I'm no acoustic engineer, but I've dabbled in the issues of sound (especially as it relates to worker exposure). Sound is a very complex and often, non-intuitive discipline. I know just enough to be consciously incompetent.
If you're well-fitting windows are not blocking the sound, you may really be up against it as noise may be permeating other areas of your house - not just the windows. From personal experience, we replaced windows for energy savings and noticed an incredible lowering of obtrusive sound afterwards. Thats why I'm skeptical that it's actually your windows allowing in all the sound, but, of course, I don't know.
One other thing you might consider. It might actually be easier, more efficient and more effective to work on your sleeping quarters. If it's just you (or you and a partner) the problem becomes one room instead of perhaps a whole house. Very tight fitting and closed doors can make an incredible reduction in many frequencies of sound. You could even add insulation to your interior bedroom walls. Someone suggested grey (I call it white) noise. With a well sealed sleeping "chamber" and white noise generator, you might find the noise to be well controlled. Of course, you won't hear an intruder breaking into your house, either, but we all have to make sacrifices.
One final suggestion: Hire an acoustical engineer to look at your problem. A good one can diagnose the problem and suggest fixes (and estimate costs). It may not be cheap, but at least you won't feel "stoooopid"
for say, blocking off the windows, only to find out that wasn't the problem. Of course, and as always, YMMV.
Good luck!