Introduction

Insomnia comes from the Latin words for 'no sleep.' Insomnia is characterized by:

• Difficulty falling asleep
• Difficulty staying asleep
• Waking up too early in the morning

Some experts believe that poor quality ('non-restorative') sleep is also related to insomnia. Insomnia can cause daytime fatigue, irritability, and impaired performance. About 60 million Americans each year suffer from insomnia.

Insomnia may be primary or secondary:

• Primary insomnia means that the inability to sleep is not caused by other health problems.
• Secondary insomnia is due to other health conditions that interfere with sleep. Some experts prefer the term 'co-morbid insomnia.'

Duration of Insomnia

Insomnia, usually temporary, is often categorized by how long it lasts:

• Transient insomnia lasts for a few days.
• Short-term insomnia lasts for no more than 3 weeks.
• Chronic insomnia occurs at least 3 nights per week for 1 month or longer.

Forms of Insomnia

Insomnia may also be defined in terms of inability to sleep at conventional times. The following examples are referred to as circadian rhythm disorders:

• Delayed Sleep-Phase Syndrome. Delayed sleep-phase syndrome is the term for a circadian clock that runs late but reliably. People who have this condition (usually adolescents) fall asleep very late at night or in early morning hours, but then sleep normally.
• Advanced Sleep-Phase Syndrome. This syndrome tends to develop in older people. It produces excessive sleepiness in the morning and undesired awakening early (3 - 5 a.m.) in the morning.

Healthy Sleep

In sleep studies, subjects spend about one-third of their time asleep, suggesting that most people need about 8 hours of sleep each day. Individual adults differ in the amount of sleep they need to feel well rested, however. (Infants may sleep as many as 16 hours a day.)

The daily cycle of life, which includes sleeping and waking, is called a circadian (meaning "about a day") rhythm, commonly referred to as the biologic clock. Hundreds of bodily functions follow biologic clocks, but sleeping and waking comprise the most prominent circadian rhythm. The sleeping and waking cycle is about 24 hours. (If confined to windowless apartments, with no clocks or other time cues, sleeping and waking as their bodies dictate, humans typically live on slightly longer than 24-hour cycles.) It usually takes the following daily patterns:

• Humans are designed for daytime activity and nighttime rest.
• Additionally, there is a natural peak in sleepiness at mid-day, the traditional siesta time.

In addition, daily rhythms intermesh with other factors that may interfere or change individual patterns:

• The fraction-of-a-second-firing of nerve cells in the brain may be faster or slower in different individuals.
• The monthly menstrual cycle in women can shift the pattern.
• Light signals coming through the eyes reset the circadian cycles each day, so changes in season or various exposures to light and dark can unsettle the pattern. The importance of sunlight as a cue for circadian rhythms is dramatized by the problems experienced by people who are totally blind. They commonly suffer trouble sleeping and other rhythm disruptions.