Number the sentences according to their level of generality from 1 to 5 (1 being the most general). The topic sentence is the only sentence that is level 1.
Insomnia, which causes people to have trouble falling asleep or staying asleep, is one of the most common types of sleep disorders.
Almost everyone has experienced occasional insomnia, also called transient insomnia.
This kind of insomnia does not usually require medical intervention.
People who experience insomnia fairly regularly, though not constantly, are said to have intermittent insomnia.
Both transient and intermittent insomnia are often caused by mental or physical problems, such as pain or worry.
They may also be the result of environmental conditions, such as extreme temperatures or noise.
When these conditions disappear, the insomnia does too.
However, constant insomnia, also known as chronic insomnia, is a much more complex and difficult problem.
It can be very disruptive to people's lives, causing continual irritation, tiredness, and an inability to concentrate.
It can also be a symptom of a serious medical problem, such as heart failure or kidney disease.
For these reasons, people with chronic insomnia are advised to seek medical help.