Insomnia is a sleep disorder characterized by difficulties falling asleep, staying asleep or getting good quality sleep.

According to a 2022 review published in the journal Sleep Medicine Clinics, approximately 10 percent of the global adult population suffers from an insomnia disorder, while 20% experiences occasional insomnia symptoms.

Insomnia can impact physical and mental health. Individuals suffering from this condition can feel sleepy during the day, have difficulty concentrating and learning and may feel irritable or anxious, according to the National Institutes of Health. Sleep-deprived individuals also tend to have high levels of the stress hormone cortisol and decreased immunity, a 2022 review published in the journal Sleep Medicine Clinics reports.

According to the NIH, insomnia can be classified as short-term or long-term (chronic). Short-term insomnia may last for a few days or weeks and is usually caused by stress or changes to a person's schedule or environment. Chronic insomnia is diagnosed when the symptoms occur three or more nights a week, last more than three months and cannot be fully explained by another health problem.

The definition of insomnia has changed with the latest edition of the International Classification of Sleep Disorders, according to a 2021 review published in The Journal of Neuropsychiatry and Clinical Neurosciences. The current focus is now on the length of symptoms, rather than the causes of the condition, as this approach may better reflect the complexity of this disorder. However, many experts, including those Live Science spoke to, still use the terms primary or secondary when referring to the condition.

Insomnia: Causes

"The causes of primary insomnia could include stress related to life events such as a new job or death of a loved one," she said. "It could also be related to poor sleep habits, such as an inconsistent bedtime, eating too late at night or not having a regular routine due to shift work."