If you just cannot sleep get up and read. The inability to be able to sleep can happen to anyone. Overfatigue will sometimes produce insomnia as you may be too tired to go to sleep. A virus or run down condition can also cause you to not be able to sleep. There is also mental fatigue that you might have for example after studying too hard for an exam.
Mental fatigue can also result from boredom or from a difficult emotional experience.
Usually the best cure for mental fatigue is some form of recreation, such as painting, fishing, etc. The best cure for the physical fatigue is sleep. If you tend to delay going to sleep after retiring try taking a warm bath, or a little exercise or deep breathing or perhaps have a hot drink or a small amount of food. Be very careful with taking over the counter sleep producing products as they could be very harmful to your health and should be used very rarely. If you need to take drugs to go to sleep there could be something wrong with your nervous system and your health habits.