Insomnia is a symptom of a sleeping disorder characterized by persistent difficulty falling asleep or staying asleep despite the opportunity. Insomnia is a symptom, not a stand-alone diagnosis or a disease. By definition, insomnia is "difficulty initiating or maintaining sleep, or both" and it may be due to inadequate quality or quantity of sleep. It is typically followed by functional impairment while awake. Insomniacs have been known to complain about being unable to close their eyes or "rest their mind" for more than a few minutes at a time. Both organic and non-organic insomnia constitute a sleep disorder.