It’s hard to say how many people have insomnia because (1) different people have different definitions of insomnia and (2) the condition is under-reported and underdiagnosed.
If you read the scientific literature, estimates of the prevalence of insomnia are all over the place. So the problem is not that the lay public doesn’t understand the formal definitions of insomnia; it’s that there are differing data-gathering techniques even among the professionals.
Our assessment: although 30% of the adult population has some bouts or symptoms of insomnia, it is estimated that 10% fit into the formal definition of insomnia with excessive daytime sleepiness and the condition lasting at least a month. Further, if you remove other sleep disorders, mental and other physical disorders, and side effects of medicines or substance abuse, the number falls to 6%.
However the NIH Sleep Disorders Research Plan (2011) states “Chronic insomnia affects nearly one out of five adults”.
Some observers feel the prevalence of insomnia is increasing over time, but there is little hard evidence for this allegation. Even if the prevalence is not increasing, the widespread misery it causes is enough to warrant considering insomnia a public health problem. Insomnia reduces the quality of life for millions, causes accidents and workplace absenteeism, and more health care costs.
There is a positive relationship between incidence and prevalence, but in diseases or illnesses that disappear quickly, it is possible for the incidence rate to be high and the prevalence rate to be low. For insomnia, prevalence is more relevant for chronic insomnia than for acute insomnia.
When you ask a large group of people if they have insomnia many will answer "yes" - more than actually have insomnia by a formal definition. Self-reporting is notoriously unreliable for getting accurate numbers for public health. Polls show 95% of adults report experiencing insomnia at some point in their life and 33% report insomnia in the previous year. http://primarypsychiatry.com/introduction-to-chronic-insomnia-classification-and-prevalence/ Of course, self-reporting in notoriously unreliable.
The National Institutes of Health estimate that 26% of American adults have some sort of mental illness at one point over the course of a year. Estimates like this are all over the place and depends a lot on definition and self-reporting. A European organization says 38% of Europeans http://www.nature.com/news/2011/110905/full/news.2011.514.html have some sort of a mental disorder and that includes 7% with insomnia.
It depends on the definition of insomnia, of course. The large majority of individuals have at least the occasional bad night's sleep, and in a folk or day-to-day understanding of sleep issues, that means they have insomnia. But any formal definition of insomnia includes some requirement for daytime impairment. According to Dr. Thomas Roth, incorporating this requirement reduces the prevalence to 10%. Adding the even more stringent requirement that the symptoms persist for a month, the prevalence drops to 6%.
What makes insomnia a "disorder"? A condition is a disorder if it has negative consequences and if those consequences are a pathological response rather than a normal part of the condition. Insomnia is a disorder and also a symptom and also a precursor/cause. The folk definition of insomnia - a bad night's sleep - does not necessarily constitute a disorder. But under the formal diagnosis created by medical associations, insomnia is indeed a disorder.