The study in question looked at the effects of aerobic exercise in insomnia sufferers over the age of 55 (most were women). Participants were divided into aerobic activity groups and non-aerobic activity groups.
Researchers found that after sixteen weeks, participants in the aerobic activity group reported improved sleep quality, fewer depressive symptoms, increased vitality and less daytime sleepiness.
It would appear that all exercise is beneficial (yoga can be a big help) - but nothing beats aerobic exercise when it comes to improving your sleep.
Source: Northwestern University