Stress is a big factor
Stress is a big factor, if you are worried about deadlines, or family issues, then these will play on your mind and continue to roll around in your thoughts long after your head hits the pillow. Another reason is noise. If you live in a busy neighborhood, near a main road, or just have disrespectful neighbors, then the constant noise will be enough to keep even the heaviest of sleepers awake.
One thing that we do have control over is the amount of light that comes into our bedroom, and this can be a big contributing factor to our inability to sleep. Getting a blackout blinds for your bedroom windows will greatly alleviate this problem, and they are not all that expensive to set up, considering the benefits you will get from them.