The findings of the study actually make a lot of sense when you think about it. It appears that "in the short term, sleep may have more of an impact on exercise than exercise has on sleep." Meaning that you're not very likely to have a good workout after a night of bad sleep, while a good workout didn't necessarily mean that you would sleep better. Since people with insomnia have what the author of the study calls "a hyper-arousal of the stress system," — going to the gym isn't enough to override the stress. It could actually potentially make it worse, since "exercise itself is a physical stressor."
So, does exercising help insomnia at all? Yes, it just takes time — four months, the study found. And, after four months of consistent exercise, the improvements in sleep were drastic. People were sleeping as much as 45 minutes more each night. So, it seems that when it comes to the exercise cure for insomnia, you just have to stick it out. Don't most hard things have to get worse before they get better, anyway? ( The New York Times )