Cannabis increases your body's natural melatonin levels up to 5,000%, depending on your tolerance. Melatonin is a sleep hormone that makes you relaxed and sleepy, hence the burnout at the end of your high.

If you smoke regularily, some people's bodies stop producing adaquate amounts of melatonin because the cannabis is already doing it for you. This is generally how it goes with frequent use of any drug that takes over a function your body would normally perform.
If you want to get some sleep, abstain from the weed completely and after 3-4 days you should be back to normal, producing the right chemicals once again. If you can't wait that long, or don't want to quit, you can buy melatonin supplements at most healthfood stores. Legality of the hormone is varried in some areas, but for the most part it's not scheduled in any way shape for form.