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While insomnia is the most common sleep disorder, a lot of attention in the media is focused on sleep apnea because of its association with cardiovascular disease and diabetes. Variations in prevalence from one country to another have been found and data regarding the prevalence of sleep apnea has only been available since the 1990s but the data indicate that across countries 3 to 7% of men and 2 to 5% of women have sleep apnea (Punjabi, 2008). In the US the prevalence rate is 4% of men and 2% of women (Punjabi, 2008). To be diagnosed with sleep apnea there must be repeated breathing pauses during sleep that are accompanied by daytime symptoms like excessive drowsiness. Given the probable under diagnosis of and difficulty of adequately accounting for all cases of sleep apnea, it is likely that the above figures are an underestimate of the true prevalence of the illness.
By comparison, about 30% of the population of the US has at least some symptoms of insomnia. Insomnia that is severe enough to be associated with daytime consequences such as memory difficulties, lack of focus, or low mood, affects about 10% of the US population at any given time (Roth, 2007). Chronic insomnia that has lasted for at least one month and is not due to another sleep disorder, another medical disorder, or the effect of a substance, has a prevalence of about 6% (Roth, 2007). There are far more people suffering from insomnia than sleep apnea, but is insomnia as destructive to health as sleep apnea?
Insomnia is a disorder of over arousal and is different from the problem of insufficient sleep. Insufficient sleep is getting too little sleep because of scheduling too little time for sleep or, for example, regularly working long shifts that prevent getting enough sleep. The person with insufficient sleep will easily sleep if the opportunity presents itself. Insomnia, on the other hand, is characterized by difficulty falling or staying asleep and results in daytime symptoms such as memory problems or low mood that interfere with significant activities such as work or social obligations. Treatment of insufficient sleep involves getting enough sleep, which can, of course, be a lot more difficult than it sounds. Treatment of insomnia involves interventions such as reducing stress related to excessive thinking at night, or worry about lack of sleep, or extinguishing conditioned arousal to being in bed.
Fortunately, insomnia and insufficient sleep are not directly causes of death. Poor attention or brief “microsleeps” can occur with both of them, however, and can cause accidents such as car crashes or falls that do certainly cause serious injury or loss of life. There are no recorded cases of death caused directly by insomnia and our bodies are remarkably capable of functioning despite significant sleep loss. One of the approaches used in cognitive behavioral therapy of insomnia is to reassure often frightened patients that insomnia is not fatal and the body can cope with and function relatively well despite getting less than optimal sleep. Worry about the health consequences of insomnia only serves to increase arousal and lead to more worry and poorer sleep. But does that mean that there are no physical consequences of having chronic insomnia?
Insomnia does have considerable documented comorbidity with psychiatric illness. Perhaps as many as 40% of patients with chronic insomnia have a psychiatric disorder, most often depression. In fact, insomnia is a diagnostic criterion for both depressive and anxiety disorders (American Psychiatric Association, 2013). People with insomnia have 2.5 to 4.5 times more accidents than those without insomnia and have total health care costs that are 60% higher than for patients without insomnia (see Roth, 2007). Interestingly, insomnia may function as a kind of “sentinel event” that frequently precedes rather than follows the onset of psychiatric illness. Anyone who has dealt with bipolar disorder, either themselves or in their loved ones, is most likely aware of the significance of the onset of a bout of insomnia.
What has become increasingly clear over the past decade is that the physical health consequences of insomnia are, in the long term, not benign. Javaheri & Redline (2017) reviewed the existing evidence for the relationship between insomnia and heart disease and some of their findings follow. Studies have indicated that insomnia is comorbid with and may increase the risk for high blood pressure, coronary heart disease, and heart failure. This is especially true when insomnia occurs in the setting of short sleep of less than six hours per night. It may be that insomnia and short sleep each contribute to the risk of disease or it may be that each increases the negative impact of the other, perhaps by further increasing the activation of the body’s stress system. The studies are limited because different definitions of insomnia have been used, such as how long it must persist before being considered chronic, and by how it has been measured, whether with questionnaires, sleep logs, or overnight laboratory sleep studies with objective measurements. Any differences in definition or measurement method may yield somewhat different results. Despite these limitations, there are now sufficient data available to believe that insomnia is a risk factor for cardiovascular disease.
The relationship between sleep apnea and cardiovascular disease is most likely related to the repeated episodes of decreased blood oxygen levels as breathing is cut off by the collapse of the upper airway and the repeated stimulation of the heart as the brain arouses the body to increase the tone of the airway muscles so that breathing can be restored. Over time this takes a toll on the heart and blood vessels. The ways in which insomnia may negatively affect the vascular system are not as well understood but most likely involve processes such as increased sympathetic nervous system activity associated with over arousal and increased inflammation.
As noted above, insomnia is due to over arousal that persists into the night. This means that the sympathetic nervous system has increased activity with increased release of stress hormones such as cortisol. Chronic activation of the stress system may have effects such as increasing insulin resistance and contributing to psychological problems such as depression and anxiety. Depression itself can increase the risk of cardiovascular disease. Insomnia appears to be associated with impaired glucose metabolism and this may damage the cardiovascular system and contribute to the development of cardiovascular disease. Dysregulation of the sympathetic nervous system has been linked to high blood pressure, cardiac arrhythmias, and heart failure. Increased levels of C-reactive protein have been linked to severe insomnia in men. Elevated levels of C-reactive protein have been implicated in the development of plaques in arteries. Very chronic insomnia has been associated with increased risk of high blood pressure. This is especially the case when patients with insomnia also get less than six hours of sleep per night.
A model of the potential relationship between insomnia and cardiovascular disease focuses on the conditioned over or hyperarousal involved in insomnia along with the potential effect of short sleep duration. This state of hyperarousal involves increased sympathetic nervous system activity, increased systemic inflammation, and dysregulation of the hypothalamic-pituitary-adrenal axis. Increased sympathetic activity feeds back on and contributes to the state of over arousal in insomnia. These factors then impact on various pathways to cardiovascular disease such as increasing heart rate and blood pressure, increasing the development of plaques in the arteries, and increasing resistance to insulin and raising levels of lipids such as cholesterol in the blood.
Clearly, more research is needed in this area. Important areas for future work include better understanding of the relationship between insomnia and cardiovascular disease and the degree to which treating insomnia reduces risk of or improves the lives of patients suffering with it. This will require developing consistent ways of defining insomnia and better methods of accurately measuring it. What is clear is that that the relationship between sleep and mental and physical health is extremely important.
American Psychiatric Association (2013). Diagnostic and Statistical Manual of Mental Disorders, 5th Ed. Arlington, VA: American Psychiatric Association.
Javaheri, S. & Redline, S. (2017). Insomnia and risk of cardiovascular disease. Chest, 152 (2), p. 435 – 444.
Punjabi, N. M. (2008). The Epidemiology of Adult Obstructive Sleep Apnea. Proceedings of the American Thoracic Society, 5(2), 136–143. http://doi.org/10.1513/pats.200709-155MG
Roth, T. (2007). Insomnia: Definition, Prevalence, Etiology, and Consequences. Journal of Clinical Sleep Medicine : JCSM : Official Publication of the American Academy of Sleep Medicine, 3(5 Suppl), S7–S10.
