<human>:  Why do some people have insomnia and others don’t?
<bot>: Insomnia is a sleep disorder that is characterized by difficulty falling asleep or staying asleep. It is the most common sleep disorder, affecting about 30% of the population. There are many possible causes of insomnia, including stress, anxiety, depression, and certain medications. Insomnia can also be caused by a variety of medical conditions, such as sleep apnea, restless leg syndrome, and certain medications.