One way to consider insomnia is not a lack of sleepiness, but too much wakefulness – hyperarousal. Hyperarousal can result from many causes, of psychological and physiological origin. It’s a heightened awareness both day and night – being "high-strung" – and scientists have shown that chronic insomniacs have a higher metabolic rate than good sleepers. So in this model, each person has two forces in them – a drive toward sleep and a drive toward arousal – and the problem in insomnia isn’t that the drive toward sleep is too weak, it’s that the drive toward arousal is too strong. There is evidence that people with insomnia have longer times on the MSLT during the day (i.e. it takes them longer to fall asleep) despite complaining of sleepiness This fits with the model of too much alertness. Some researchers even created a Hyperarousal Scale and it was found that insomniacs tend to score high on it. The brains of insomniacs also tend to use more glucose.
Although the question is not settled, evidence continues to accumulate that insomnia involves high activity in both the sleep and arousal systems. Post-traumatic stress disorder patients also have high rates of insomnia that seem due to hyperarousal. Advanced medical imaging has shown that wholebrain metabolic activity is higher in people with insomnia in both daytime and night. Further the frontal lobes of the brain are particularly active during waking in insomniacs.
And what is this "arousal"? Arousal – the energetic state at any time - is a complex physiological state influenced by endocrine and neurologic systems. Stress hormones are part of it, and the largely unknown workings of the central nervous system. The recently discovered orexin system is part of the arousal/waking circuit. Another neuropeptide, melanin-concentrating hormone (MCH), is produced in the lateral hypothalamus portion of the brain, along with orexin. Like so many bodily phenomena, arousal has a homeostatic component to it – a moderating and smoothing influence – even if levels of arousal change over the course of the day. The environment has a great deal of influence on a person’s arousal level, and many neurotransmitters, cytokines, and hormones are part of the system.
Epidemiologists speak of "risk factors" for diseases and illnesses. Risk factors for insomnia is almost a meaningless term because insomnia is so common and because there are so many causes and co-morbidities. Any list of risk factors would include advanced age, being a woman, life changes (e.g. losing a job), mental illness, high alcohol and caffeine consumption, other illnesses, chronic pain, and side effects of various medications.
When risk factors are in place, precipitating factors can start an insomnia episode. Precipitating factors can be as simple as a change in daylight savings time or work schedule. Often it is a stressful event the person experiences in his or her work, school, or family life – so-called psychosocial stressors. Stress is a common cause of short-term insomnia, and the short-term sleep loss can ignite a cycle of long-term insomnia. Medication can be a precipitating factor as can other illnesses – all the causes of secondary insomnia are in this category.
Another paradigm sometimes used among sleep therapists is the 3P model: Predisposing, Precipitating, and Perpetuating Factors. Perpetuating factors are patterns and behaviors that form during the insomnia and which paradoxically keep the person from returning to good sleep, even after the precipitating factors are gone. For instance, anxiety about getting to sleep may have developed and may persist even after the nominal cause of the insomnia disappears. Bad sleep habits may be in place, or the insomniac may have started taking an OTC sleep aid and feel unable to quit it, getting rebound insomnia when an attempt to quit is made. Alcohol and drug use can also perpetuate insomnia.
Practitioners of CBT often have the 3P model in mind and use it as a framework or part of a framework for addressing insomnia and getting the patient back into a good long-term sleeping pattern.
Insomnia can be a self-fulfilling prophecy. Rumination and worry can cause insomnia – along with depression – and are consequences of insomnia. Anxiety about getting to sleep in the past can contribute to difficulties getting to sleep tonight. Worry is a perpetuating factor that keeps the insomnia going. When psychological and physical causes are both involved, the result can be called psychophysiologic insomnia. This happens a lot. Maybe a physical ailment will cause insomnia; when the ailment clears up the mind still enforces the insomnia because it has learned to not sleep at certain times or in a certain place. Worry about sleeping and a shift in the time actual sleep is obtained (causing de facto jet lag) make it all the harder to get back on track. It’s a self-reinforcing system.
Insomnia appears to have a genetic predisposition, too, although the causes of insomnia are so varied and sleep and waking are so complex, that it is hard to tease out what is due to what. A classic study of twins estimated that the insomnia heritability was 57% and sleepiness heritability was 38%. These numbers should be taken with a grain of salt when it comes to blaming your insomnia on your genes.
Recent analysis suggests that observable symptoms of insomnia manifest when people are awake may be related to the source of the insomnia. Secondary insomnia that can be traced to a mental disorder as well as unknown-origin "idiopathic insomnia" results more often in daytime mood disturbances, while psychophysiological insomnia is more often associated with poor sleep hygiene and tension and low daytime fatigue. Paradoxical insomnia, often called sleep state misrepresentation, resulted in little daytime impairment, while mental health related insomnia typically have high daytime impairment.
Older people have much greater rates of insomnia. This used to be thought normal, but after much arguing and detailed research, the consensus is now that very healthy seniors are not doomed to poor sleep and that insomnia is not an inevitable consequence of age. The fact is that older people have more ailments – diagnosed and undiagnosed, blatant and subtle – than young people and these may affect in myriad ways tendencies to insomnia.
Also, cancer patients have high rates of insomnia; the etiology is not known, but many common causes of insomnia are more prevalent in people with cancer. These include depression, worry, and pain.