20 Minutes a day can make all the difference.
Studies have shown that if you engage in at least 20 minutes of exercise per day that it can help improve sleep. Expending energy on a daily basis will help the duration of sleep (so maybe letting you sleep longer), reduce stress levels which can wreak havoc on sleep, and there have now been some studies done that aerobic exercise can help insomnia.
So whether it's running, weights, a pool work out, or even some yoga, get out there and work!