A normal nights sleep has three main parts.
Each night, about 4-5 periods of quiet sleep alternate with 4-5 periods of REM sleep. In addition, several short periods of waking for 1-2 minutes occur about every two hours or so, but occur more frequently towards the end of the nights sleep. The graph below shows a typical normal pattern of sleep in a young adult.
Normally, you do not remember the times that you wake if they last less than two minutes. If you are distracted during the wakeful times (for example, a partner snoring, traffic noise, fear of being awake, etc), then the wakeful times tend to last longer, and you are more likely to remember them.
Insomnia means poor sleep. About 1 in 5 adults do not get as much sleep as they would like. Poor sleep can mean:
If you have poor sleep, you may be tired in the daytime, have reduced concentration, become irritable, or just not function well.
Different people need different amounts of sleep. Some people function well and are not tired during the day with just 3-4 hours sleep a night. Most people need more than this. To need 6-8 hours per night is average. Most people establish a pattern that is normal for them in their early adult life. However, as you become older it is normal to sleep less. Many people in their 70s sleep less than six hours per night.
So, everyone is different. What is important is that the amount of sleep that you get is sufficient for you, and that you usually feel refreshed and not sleepy during the daytime. Therefore, the strict medical definition of insomnia is "a persistent difficulty falling or staying asleep, leading to impairment of daytime functioning".