Do something tiring before bed. Do push ups, crunches, stuff like that.
That actually makes it worse. It quickens your heart rate and keeps you awake for longer. The original crash will make you think you're tired, but you'll lie awake for far longer than originally planned.
Take this stuff:
It is a hormone naturally produced by the pineal gland in the brain. It regulates your day-night cycle and causes you to fall asleep faster and sleep more deeply. It is also non-addictive and contains no serious side-effects from long-term use.
I use it often. It works very well.
