The basic idea of insomnia is that people will be staying awake at night without any reason. People think that it is caused by stress or simply not being able to get to sleep. Sometimes they will not be able to rest because of eating at different points of the day.
No matter what the reason is, the fact is that there are times when people are not able to get back to sleep.
Insomnia by definition is the difficulty of falling asleep, or it could be the difficulty of staying asleep. Both of these definitions have to do with the fact that the person is having trouble getting to sleep since they are facing another kind of troubling situation mentally.
A person will know that they have insomnia when they are not satisfied with the amount of sleep that they have been getting.