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Insomnia is difficulty falling asleep or staying asleep, resulting in inadequate length of sleep and/or poor quality of sleep. The disturbances caused by insomnia affect people during their waking hours.

People with insomnia may wake frequently during the night and have difficulty falling back asleep or may wake up too early in the morning. Insomnia is the most common type of sleep disorder.

Sleep requirements differ among individuals. Therefore, insomnia is not defined by how long it takes to fall asleep or the total number of hours spent sleeping. Most adults require seven to eight hours of sleep, but some people need only four to five hours.

Primary insomnia is difficulty with sleep that is diagnosed after other underlying causes (such as medications or diseases) have been either ruled out or treated. Factors such as chronic stress, hyperarousal, poor sleep habits (such as drinking caffeine before sleeping) and behavioral conditioning may contribute to primary insomnia. If insomnia can clearly be attributed to an underlying cause, it is called secondary insomnia. However, when another medical or mental health disorder is also present, it can be difficult to determine whether the insomnia is due to the disorder or if the other disorder is secondary to insomnia. In addition, some people report having insomnia that then cannot be found by a sleep study.

Insomnia may be:

• Transient. Lasting for a few weeks. Most people experience transient insomnia at some point in their lives. It is a common response to jet lag or stressful situations, such as job loss or death of a loved one.
  
  
• Intermittent. Episodes of transient insomnia that occur from time to time.
  
  
• Chronic. Insomnia occurs on most nights or lasts a month or longer. This is often the result of a medical, neurological or psychiatric disorder or other factors.

Research has shown that insomnia can be linked to problems with immune system functioning, muscle endurance and hormones. Insufficient sleep may raise the risk of obesity and type 2 diabetes, according to several recent studies. And it increases the risk of accidents such as car wrecks.

About one in three American adults experience insomnia in a given year, according to the National Institutes of Health. About one in 10 experience insomnia that is chronic or severe. Insomnia is more common among women (especially after menopause) and the elderly. About half of people over age 65 have frequent sleep problems.

Children also experience insomnia, for many of the same reasons as adults, such as stress or poor sleep habits. They can also experience insomnia as a result of nightmares and night terrors.

Changes that occur with age and may affect sleep include:

• Sleep pattern changes. Sleep becomes less restful after age 50. More time is spent in the earlier, transitional stages of sleep than the later stages (deep sleep). The later stages are the most restful kind of sleep.
  
  
• Activity changes. Older adults are less active than younger adults and activity helps facilitate a good night's sleep. Older adults may also have more free time than younger adults and may have habits that interfere with sleep, such as daytime napping and caffeine consumption.
  
  
• Health changes. Chronic pain conditions that may interfere with sleep, such as arthritis or back problems, occur more frequently with age. Also, sleep disorders that result in insomnia, such as sleep apnea (when breathing stops periodically throughout the night) and restless legs syndrome (unpleasant sensations in the legs during the night), increase with age.