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According to a population based study, severe insomnia can last about 4 years.

-In population-based studies severe insomnia has been shown to last for a median of 4 years. http://onlinelibrary.wiley.com/doi/10.1002/da.10151/abstract

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-Chronic insomnia occurs in people who have persistently high levels of stress hormones and a shift in the levels of certain immune factors.

-These immune factors, called cytokines, cause symptoms of fatigue.

People with high levels of stress hormones or shifts in the levels of cytokines are more likely to have Chronic insomnia.

http://health.nytimes.com/health/guides/symptoms/sleeping-difficulty/causes-of-chronic-insomnia.html