Is insomnia a disease or a disorder or an illness? It’s certainly an illness because anything that is uncomfortable or bothersome counts as an illness. Some illnesses are also diseases. On the question of whether insomnia is a disease or a disorder, we come down on the side of disorder, at least for primary insomnia. Some diseases are caused by infectious agents or parasites; some are due to lack of adequate nutrition or genes.
A disorder is a functional abnormality. Disorders are sometimes kicked off by a pathogen or external trauma, but there does not need to be an infectious agent to keep the disorder going.
Diseases can often be cured. Disorders can usually not be cured, although medicine often provides techniques for managing them. This meshes with our experience of insomnia, which can be treated but often returns. Sometimes the insomnia does disappear after treatment, but nobody thinks the sleeping pills cure the insomnia. The insomnia just disappears, while the pills help the person get through the rough patch.
In the case of secondary insomnia, when the sleep difficulties are a result of another underlying illness, insomnia can be classified as a symptom, as well as a disorder. Severe cases of secondary insomnia may be worth treating with medication or at least addressing with non-drug methods. But the insomnia itself is a symptom.
Scientists are increasingly seeing insomnia as a consequence of hyperarousal during the day. That is, the problem isn’t that people aren’t getting sleepy enough to fall asleep; it’s that they are too stimulated all the time. In the war between sleepiness and arousal, the arousal wins too often in the insomniac’s brain. (This is one reason there are hopes for arousal-killing orexin antagonist drugs.)
The hyperarousal idea isn’t just speculation. Modern imaging techniques can show how much energy the brain is using (more precisely, how much glucose is in the blood in different areas of the brain). Many (not all) insomniacs have higher energy consumption in the brain during waking and NREM sleep compared to good sleepers. Further insomniacs tend to have smaller reductions in energy use from waking to non-REM sleep in areas of the brain rich in wake-promoting neurons.
Of course, the etiology of insomnia varies from person to person. There are many causes and many ways insomnia plays out in our daily lives. So you have to take broad statements about hyperarousal and broad categories like primary and secondary insomnia with a grain of salt. Further, in long term chronic insomnia cases, the insomnia almost always changes, evolving in response to seasonal changes, medicines taken, psychological factors (rumination on how hard it is to sleep), etc.
This is one reason patients with insomnia should take charge of their sleep management. Your doctor can be involved, but of necessity he or she will not be able to monitor your insomnia as it progresses.
The regulation of sleep is still a mystery, although some of the underlying neurochemical factors have been identified. The circadian and homeostatic processes in the two-process model are valid and provide a good understanding and explanation of patterns and observed behavior. But there are other factors not included in this simple model that can disrupt sleep or lead to a pattern that looks like insomnia.