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. Approximately 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 ¡°comorbid 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 (3:00 - 5:00am) in the morning.