Insomnia and disease risk
Cognitive function
After a very poor night’s sleep, most of us have all experienced fatigue, brain fog, short temper and a lack of focus. Adequate sleep is a prerequisite for people to feel sharp, concentrate, proficiently assimilate information, and learn. Inadequate sleep negatively impacts problem-solving skills and the ability to regulate emotions, and make decisions. It also slows reflexes and impairs motor skills. As a consequence, accidental injuries are much more likely to occur and drowsiness is a major factor in motor vehicle accidents.
Sleep scientists believe that during sleep the brain is engaged in sorting and storing information arising from the part of the day when we are awake. This process is particularly important for converting information from short-term memory to long term memory. Adequate sleep is crucial for good recall. Recent research has shown that people who suffer from sleep disturbances in midlife or in their older years are more likely to develop cognitive impairment than people who usually get plenty of uninterrupted sleep.
Recently, scientists have started to link longer waking time/insufficient sleep with increased risk of cognitive impairment and a higher risk of developing Alzheimer’s disease. More information on this can be found in the next section.
Alzheimer’s disease
Alzheimer’s disease is the most common form of dementia worldwide and presents a high prevalence of sleep disturbances. It’s now increasingly being recognised that poor sleep is a hallmark of the disease.
Alzheimer’s disease is characterized by accumulation of extracellular amyloid plaques and intracellular accumulations of abnormal tau that aggregates in several parts of the brain’s cortex. It’s now known that sleep is crucial to consolidate memory and to remove an excessive build-up of these two substances both of which are forms of proteins.
It’s now increasingly becoming recognised that inadequate sleep is a key lifestyle factor in an individual’s risk of developing Alzheimer’s disease. Insufficient sleep is only one among several risk factors associated with Alzheimer’s disease.
This being the case, ensuring sufficient quality sleep alone will not be the magic bullet that prevents Alzheimer’s. Nevertheless, prioritising sleep throughout life appears to becoming a sensible approach to lowering the risk of developing Alzheimer’s disease or at least delay its onset.
A study published in 2019 in the journal Science Translational Medicine, found that older people who have less slow-wave sleep (non-rapid eye movement sleep) i.e. the deep sleep required to consolidate memories and to wake up feeling refreshed, have higher levels of the brain protein tau. Elevated tau is a sign of Alzheimer’s disease. The lead researcher commented:
“What’s interesting is that we saw this inverse relationship between decreased slow-wave sleep and more tau protein in people who were either cognitively normal or very mildly impaired, meaning that reduced slow-wave activity may be a marker for the transition between normal and impaired.”
Other research published within the last seven years included the following:
A community based prospective cohort study published in 2013 in the journal Sleep found that individuals with high sleep fragmentation had a 1.5-fold risk of developing AD as compared with someone with low sleep fragmentation.
A forty year study of men aged 50 and older, published in 2015, in the journal Alzheimer’s & Dementia, observed that those with sleep disturbance had a 51% increase in the risk of developing Alzheimer’s disease.
A systematic review and meta-analysis published in 2017 in the journal Sleep concluded that:
- Individuals with sleep problems had a 1.68 times higher risk for the combined outcome of dementia and/or Alzheimer’s disease.
- Approximately 15% of Alzheimer’s disease in the population may be attributed to sleep problems.
Cardiovascular disease
During sleep, the sympathetic nervous system, which is associated with the stress response (also known as the fight or flight response) gets a chance to relax. Non-rapid eye movement sleep (NREM) sleep is generally characterized by parasympathetic dominance, slow rolling eye movements, decreased muscle tone, heart rate, breathing, blood pressure, metabolic rate, and temperature.
However, research has shown that when we’re sleep deprived, sympathetic nervous system activity increases. This is associated with an increase in blood pressure, higher heart rate and higher levels of chemicals linked with inflammation. This puts unnecessary strain on the heart. Elevated blood pressure (hypertension) is a major risk factor for cardiovascular disease, stroke, and diabetes.
A study published in 2011 reported on a research programme which followed up evidence from 7 to 25 years from more than 470,000 participants from 8 countries. In commenting on the findings of the research, the lead researcher said:
“If you sleep less than six hours per night and have disturbed sleep you stand a 48 per cent greater chance of developing or dying from heart disease and a 15 per cent greater chance of developing or dying of a stroke.”
Another study, published in 2011, examined the effects of sleep duration on the incidence of cardiovascular events among 2,282 middle-aged male workers in Japan. The study concluded that short sleep duration (less than 6 hours) was a significant risk factor for coronary events.
A review paper published in CHEST Journal in 2017 stated the following:
“Despite some inconsistencies in the literature, likely due to variations in how insomnia is defined and measured, the existing data suggest that insomnia, especially when accompanied by short sleep duration, is associated with increased risk for hypertension, coronary heart disease, and recurrent acute coronary syndrome, and heart failure.”
A study published in 2019 in the journal Psychosomatic Medicine set out to learn more about the connection between poor sleep and cardiovascular health problems in 300 individuals with no history of heart problems. Participants wore portable blood pressure cuffs for two consecutive days. The cuffs randomly took participants’ blood pressure during 45-minute intervals throughout each day and also overnight. At night, participants wore actigraphy monitors (wristwatch-like devices that measure movement) to help determine their ‘sleep efficiency’ i.e. the ratio of the total time spent asleep in a night compared to the total amount of time spent in bed. Overall, those who had lower sleep efficiency showed an increase in blood pressure during that restless night.
A study that involved 487,200 people in China with an average age of 51 was published in 2019 in the journal Neurology. The participants had no history of stroke or heart disease at the beginning of the study. They were asked if they had any of the following 3 symptoms of insomnia at least 3 days per week: trouble falling asleep or staying asleep; waking up too early in the morning; or trouble staying focused during the day due to poor sleep.
The participants were then followed for an average of about 10 years. During that time, there were 130,032 cases of stroke, heart attack and other similar diseases. Individuals who had reported all 3 symptoms of insomnia were 18 percent more likely to develop these diseases than people who did not report any of these symptoms.
The study author commented:
“These results suggest that if we can target people who are having trouble sleeping with behavioral therapies, it’s possible that we could reduce the number of cases of stroke, heart attack and other diseases later down the line.”
Immune system
During sleep, the immune system releases a type of small proteins called cytokines. If you’re sick or injured, these cytokines help your body fight inflammation and infection. Without enough sleep, the immune system might not be able to function at its best. A prolonged lack of sleep causes a similar reaction to high levels of stress; it can decrease your antibody response. So a good night’s sleep helps to strengthen your body’s immune response.
According to the findings of a study published in the Journal of Experimental Medicine in 2019, sleep enhances the ability of T cells to attach to their targets. T cells are a type of white blood cells that are the foundation of the human body’s immune system. Large quantities of T cells are present in the bloodstream and are ready to attack viruses and other pathogens that invade the body.
Type-2 diabetes
Type-2 diabetes is more common in sleep-deprived individuals. This is thought to be due to a slower processing of glucose (which the body uses for energy) than occurs in those with normal sleep. Therefore, it seems that good sleep can help prevent diabetes. Studies have suggested that people who usually sleep less than 5 hours a night have an increased risk of developing diabetes.
Among the findings of a study presented at the annual meeting of the Endocrine Society in 2018, was that even one night of restricted sleep can cause insulin resistance. Insulin resistance is the name given to when cells of the body don’t respond properly to the hormone insulin. Insulin resistance is the driving factor that leads to type 2 diabetes, gestational diabetes and prediabetes
According to the findings of a study published in 2017 in the journal Sleep Medicine Reviews, lack of sleep among pregnant women, may be a contributing factor to the development of gestational diabetes. Women who have gestational diabetes are at an increased risk for developing type 2 diabetes later in life. The babies of these women are also at an increased risk for type 2 diabetes and obesity. Gestational diabetes most often occurs in the second or third trimester.
Obesity
Studies have shown that people who sleep less than seven hours a day tend to gain more weight and have a higher risk of becoming obese than those who regularly get seven hours sleep. This is thought to be due to the fact that sleep-deprived individuals have reduced levels of leptin and increased levels of ghrelin. Leptin is a hormone secreted from fat cells that helps to regulate body weight by signalling to the brain that there is enough stored fat, thereby curbing appetite. Ghrelin is a hormone produced and released mainly by the gastrointestinal tract. It is called the ‘hunger hormone’ because it stimulates appetite. Therefore, not getting enough sleep can make it difficult to control appetite and result in weight gain. So sleep plays a key role in regulating how the body uses food for energy and getting enough sleep can help with weight control.
A study published in 2019 by the NIH/National Heart, Lung and Blood Institute found that not sticking to a regular bedtime and wake up schedule, and getting different amounts of sleep each night, can put a person at higher risk for obesity, high cholesterol, hypertension, high blood sugar and other metabolic disorders.
Mental health
Sleep and mental health are closely connected because sleep deprivation affects an individual’s psychological state and mental health. Traditionally, clinicians treating patients with psychiatric disorders have viewed insomnia and other sleep disorders as symptoms. But studies in both adults and children suggest that sleep problems may raise the risk for, and even directly contribute to, the development of some psychiatric disorders. This research has clinical application, because treating a sleep disorder may also help alleviate symptoms of a co-occurring mental health problem.
Studies suggest that a good night’s sleep helps foster both mental and emotional resilience, while chronic sleep deprivation sets the stage for negative thinking and emotional vulnerability. Most people feel irritable if they haven’t had a good night’s sleep, but long-term sleep deprivation has been linked to clinical depression and a more general loss of motivation. Also, patients with depression often have irregular sleep schedules.
Anxiety and panic attacks are also common in individuals who have chronic sleep deficiency. They have also been shown to have a lower tolerance for even mild daily stressors. Sometimes it can be difficult to understand what came first i.e. the anxiety or the sleep disorder.
According to a study published in 2018 in the Journal of Behaviour Therapy and Experimental Psychiatry, sleeping less than eight hours per night is associated with intrusive, repetitive thoughts like those seen in anxiety or depression.
According to the lead researcher:
“We found that people in this study have some tendencies to have thoughts get stuck in their heads, and their elevated negative thinking makes it difficult for them to disengage with the negative stimuli that we exposed them to.”
“While other people may be able to receive negative information and move on, the participants had trouble ignoring it.”
“These negative thoughts are believed to leave people vulnerable to different types of psychological disorders, such as anxiety or depression.”
Male infertility
A small study published in 2011 in the Journal of the American Medical Association (JAMA), reported that healthy young men who slept less than five hours a night for one week in a laboratory had significantly lower levels of testosterone than when they had a full night’s sleep. This study also found that skipping sleep reduces a young man’s testosterone levels by the same amount as aging 10 to 15 years. The effects of sleep loss on testosterone levels were apparent after just one week of short sleep. Five hours of sleep decreased the young men’s testosterone levels by 10% to 15%. Testosterone levels in men usually decline by 1% to 2% a year as they age. Testosterone deficiency is associated with low energy, reduced libido, poor concentration, and fatigue.
A study published in 2017 in the journal Environmental Pollution reported that exposure to a noisy environment at night, is linked to infertility in men. The researchers found that exposure to 55 decibels (similar to the noise from a busy suburban street) is linked to a significant increase in infertility. Male infertility is defined as a male’s inability to cause a pregnancy after having regular intercourse with a fertile female without birth control for one year.
According to World Health Organisation/Europe’s guidelines, annual average night exposure should not exceed 40 decibels, corresponding to the sound from a quiet street in a residential area. Persons exposed to higher levels over the year can suffer from sleep disturbance and insomnia. In addition, according to WHO/Europe, long-term average exposure to levels above 55 decibels can also trigger elevated blood pressure and heart attacks.
Cancer
Colorectal cancer
A study published in 2011 in the journal Cancer reported that individuals who averaged less than six hours of sleep at night had an almost 50 percent increase in the risk of colorectal adenomas compared with individuals sleeping at least seven hours per night. Adenomas are a precursor to cancer tumours, and left untreated, they can turn malignant.
A study published in 2012 in the journal Breast Cancer Research and Treatment found an association between insufficient sleep and biologically more aggressive breast tumours as well as likelihood of cancer recurrence.
The researchers found that women who reported six hours or less of sleep per night on average before breast cancer diagnosis had higher recurrence scores. The researchers commented:
“This is the first study to suggest that women who routinely sleep fewer hours may develop more aggressive breast cancers compared with women who sleep longer hours,”
“Effective intervention to increase duration of sleep and improve quality of sleep could be an under-appreciated avenue for reducing the risk of developing more aggressive breast cancers and recurrence.”
Breast cancer
A study published in 2014 in the journal Sleep reported that sleep efficiency, a ratio of time asleep to time spent in bed, is predictive of survival time for women with advanced breast cancer. The results showed that higher sleep efficiency was significantly associated with lower mortality over the ensuing six years. The researchers commented:
“Good sleep seems to have a strongly protective effect, even with advanced breast cancer.”
Prostate cancer
Men who reported sleep problems, including difficulty falling asleep and staying asleep, had up to a twofold increased risk for prostate cancer, according to data published in 2013 in the journal Cancer Epidemiology, Biomarkers & Prevention. After the researchers adjusted for age, they found that compared with men who reported no problems with sleeping, the risk for prostate cancer increased proportionately with reported severity of problems falling and staying asleep, from 1.6-fold to 2.1-fold.
Mortality
Some research has indicated that insomnia may be associated with mortality. However, the research findings have been inconsistent.
A connection between persistent insomnia and increased inflammation and mortality was identified in a study, published in 2015 in The American Journal of Medicine. The researchers found that people who suffer from persistent insomnia are at greater risk than those who experience intermittent insomnia.
According to research findings presented in 2010 at the annual SLEEP conference of the Associated Professional Sleep Societies, the results indicated that the adjusted hazard ratio for all-cause mortality was three times higher in people with chronic insomnia than in people without insomnia.
In contrast to the above, other research papers have reported no association between insomnia and risk of mortality. These include the following:
A meta-analysis review of 17 studies that evaluated the strength of evidence for a relationship between risk of mortality and frequent and ongoing insomnia was published in 2019 in Sleep Medicine Reviews. The authors reported that there was no difference in the odds of mortality for those individuals with symptoms of insomnia when compared to those without symptoms. They also reported that their analysis revealed a tendency for an increased risk of mortality associated with hypnotic use. Hypnotics are a sleep-inducing class of drugs used to treat insomnia.
A prospective cohort study published in 2019 in the journal Nature and Science of Sleep investigated the association between sleep duration, insomnia symptoms and mortality in 39,139 participants with a mean follow-up time of 19.6 years. Overall, no association was found between insomnia symptoms and mortality. However, when the analysis was stratified by sleep duration, both difficulties initiating sleep and daytime sleepiness were significantly and independently associated with mortality among those with sleep duration of 9 hrs or longer.