Because your not tired
It depends. Is this on a regulat basis? If it is so, just take some mediaction that you can directly get from the pharmacy, they'll help you sleep. If they don't work, go see a doctor.
If it's only tonight, drink green tea, or herb tea. Try listening to some music, or to the radio. Maybe you're nervous about something? Try counting endlessly.
You or thinking about something that happend in the past. Or not comfortable in side your bed.
There are many reasons for this. Anxiety, stress, and depression are some of the most common causes for this problem. Also, I would suggest you to change your mattress if you are unable to sleep well. The mattress plays a great role in providing quality sleep. Try the best latex mattress from Top Natural Mattresses. I am sure it will be helpful for you. You can also go for meditation.