People with chronic insomnia do not necessarily respond to a single treatment type. In an article in the June 2010 edition of the Journal of Clinical Sleep Medicine, the authors studied the effect of exercise on chronic insomniacs that were healthy otherwise and did not exercise regularly. They found that moderate intensity aerobic exercise decreased anxiety and improved sleep quality, when studied in the sleep lab and when they asked the participants. There was no significant improvement in anxiety or sleep quality with high intensity aerobic exercise or moderate intensity resistance exercise (weight training).
Before you go out and hit the running trail, some caution is advised. This study was small and only measured participants' response after one session of exercise. Also, the moderate intensity aerobic exercise group had the most severe insomnia to begin with - and therefore had the most to gain with the exercise session.
Exercise can be an important part of healthy sleep. Exercise too close to bedtime can disrupt sleep, especially in the beginning of the night. You might struggle more if you go for a run at 10 pm, then grab a shower, and hit the sheets by 11 pm. You'll probably need more wind-down time - perhaps 4-6 hours after exercise, before trying to sleep.