The Health Benefits of Sleep
Getting sufficient and good quality sleep is associated with important health benefits, such as boosting our immune system, maintaining our metabolism (the chemical reactions in our body), and learning and memorizing information efficiently. Most adults need 7–8 hours of uninterrupted sleep each night.1 However, approximately one in three adults report that they routinely do not get enough sleep.2 The inability to get sufficient sleep is known as sleep deprivation.
Sleep deprivation has been associated with serious health consequences, the most alarming of which is increased all-cause mortality. Interestingly, excessive sleep may also be associated with negative health consequences, including increased all-cause mortality.3 Not only the quantity but also the quality of sleep is important because frequent interruptions interfere with the normal sleep cycles. Thus, to benefit from the positive health effects of sleep, both optimal sleep duration and good sleep quality are required. Sleep deprivation has been linked to several serious health conditions, including obesity, diabetes mellitus type 2, depression, and heart disease.
Heart disease, which is the number one global cause of death, encompasses different disorders. The most common heart disease is ischemic heart disease, which is also known as coronary heart disease and coronary artery disease, and which can lead to a heart attack. In ischemic heart disease, the blood vessels that bring oxygen to the heart are narrowed, interfering with the oxygen supply to the heart. Other heart diseases are, for example, heart failure and arrhythmia. Heart failure refers to a condition in which the heart is not able to pump sufficient blood in the blood vessels, whereas in arrhythmia, the heartbeat is irregular, or the heart rate is outside of the normal range. The symptoms of heart disease may not appear right away, and it is then called “silent” heart disease.
Ischemic heart disease is the heart disorder responsible for the highest number of deaths (8.9 million global deaths in 2019).4 It has also been associated with high societal costs related to disability. Risk factors for ischemic heart disease include high blood pressure, smoking, high cholesterol (“fat”) levels in the blood, obesity, unhealthy diet, excessive alcohol use, and physical inactivity. Interestingly, sleep deprivation has also been linked to an increased risk of ischemic heart disease.
Sleep Deprivation and Cardiovascular Health (Health of the Heart and Blood Vessels)
Sleep deprivation has been associated with increased morbidity and mortality from cardiovascular diseases. A large study found that very short sleep duration and highly fragmented sleep are associated with an increased risk for subclinical atherosclerosis.5 Atherosclerosis is the accumulation of cholesterol, fats, and other compounds on and in the walls of the blood vessels that carry oxygen and nutrients to the organs (arteries). This causes the formation of atherosclerotic plaques, which can limit the blood flow. Another study examined the association between sleep duration and excess heart age. The researchers used the term heart age to designate a person's predicted blood vessel age based on their cardiovascular disease risk profile. In this context, excess heart age represented the difference between heart age and chronological age. Interestingly, excess heart age was increased both in sleep deprived individuals and individuals who slept excessively.6
Sleep Deprivation and High Blood Pressure
High blood pressure (hypertension) is a very widespread health problem, affecting one in three adults. Notably, high blood pressure is an important risk factor for both ischemic heart disease and stroke. A number of studies have found an association between sleep deprivation and high blood pressure. Thus, sleep duration of 5 hours or less per night was associated with increased risk of high blood pressure in adults aged 32–59 years.7 Another study also found that sleep duration of 5 hours or less per night was associated with an increased risk of high blood pressure, especially in younger women; in this investigation obesity partially mediated the association between sleep deprivation and high blood pressure.8
Sleep Deprivation and Ischemic Heart Disease
In different studies, both sleep deprivation and excessive sleep have been linked to increased rates of ischemic heart disease and/or heart attacks. For example, in a study that analyzed the rates of cardiovascular disease in association with sleep duration in 2005 individuals, both sleep deprivation and excessive sleep were associated with an increased rate of cardiovascular disease.9 In a large investigation on adult men, there was a higher rate of symptomatic coronary artery disease in individuals sleeping less than 6 hours per night and a higher rate of diagnosed myocardial infarction (heart attack) in individuals sleeping more than 9 hours per night.10 In another study, the rate of myocardial infarction was increased with sleep deprivation, whereas the rate of coronary heart disease was increased with excessive sleep.11
Sleep Quality and Cardiovascular Health
Interestingly, not only the overall sleep duration but also certain aspects broadly related to the quality of sleep affect cardiovascular health. For example, one study found that when the sleep that older women got differed by at least 2 hours between weeknights and the weekend, factors related to the cardiovascular health were affected negatively. The authors hypothesized that this effect may occur through a misalignment of the circadian rhythm (the natural rhythm of our bodies that follows a 24-hour cycle).12 The association between other sleep characteristics and cardiovascular health has also been investigated. For example, disordered breathing during sleep, which leads to lower oxygen levels in the blood, has been associated with worse cardiovascular health.13 Disordered breathing during sleep may be caused by a medical condition known as sleep apnea. In the same study, several other sleep characteristics, such as fragmentation of the sleep or changes in the sleep architecture, were less consistent in their association with cardiovascular health than disordered breathing during sleep.13
Experimental Studies on Sleep Deprivation and Cardiovascular Function
The studies mentioned up to this point describe an association between sleep deprivation and cardiovascular function. However, experimental studies have also been conducted that, rather than describing an association, place individuals under conditions of sleep deprivation and measure the effect on the cardiovascular system. Overall, such experimental studies have corroborated the negative impact of sleep deprivation on cardiovascular function.
Some of the investigations showed a direct negative effect of sleep deprivation on the regulation of the heart and blood vessels. For example, sleep deprivation for 24 hours resulted in increases in blood pressure levels.14 In another study, continuous sleep deprivation for 40 hours resulted in changes in the blood levels of molecules affecting the function of blood vessels, which in turn led to increases in blood pressure.15 Sleep deprivation has also been shown to affect the part of the nervous system called sympathetic nervous system, which is activated under stress. Interestingly, the activation of the sympathetic nervous system was observed after chronic rather than acute sleep deprivation.15,16 The activated sympathetic nervous system leads to norepinephrine release, which in turn causes increased blood pressure levels and heart rate. Another interesting observation is that in individuals subjected to severe sleep deprivation (less than 4 hours sleep duration per night), the blood flow in the blood vessels that bring oxygen and nutrients to the heart (coronary circulation) was affected negatively.17 Sleep deprivation has also been associated with changes in the stiffness of the blood vessels that deliver oxygen and nutrients to the different organs of the organism (arteries); however, the changes in arterial stiffness have not been consistent among all different study designs.18,19
Besides the described above direct effects on the regulation of the heart and blood vessels, sleep deprivation may also increase the cardiovascular risk indirectly through affecting factors associated with cardiovascular disease, such as obesity, insulin resistance, and inflammation.20 Sleep deprivation has consistently been associated with weight gain, a phenomenon driven to a large extent by overeating; thus, sleep deprivation increases the risk of obesity, which is a cardiovascular risk factor. Moreover, sleep deprivation leads to impairments in the metabolism of glucose (blood sugar) and insulin (a hormone that regulates the levels of glucose); thus, sleep deprivation increases the risk of diabetes mellitus type 2, another risk factor for cardiovascular disease. Sleep deprivation has also been associated with an increased concentration in the blood of molecules that promote inflammation,21 and inflammation has been implicated in several cardiovascular diseases.22
Other Factors That Influence or Explain the Association Between Sleep Deprivation and Cardiovascular Disease
Other factors, such as age, gender, and ethnicity, may affect the observed association between sleep deprivation and cardiovascular disease. Therefore, the question whether such factors can influence or explain the observed association between sleep deprivation and cardiovascular disease has been investigated. In many studies, the statistical analysis has accounted for such effects. The findings have indicated that even though certain factors may modify the association between sleep deprivation and cardiovascular disease, after controlling for them, the association may become smaller but remains significant.20
Prevention or Mitigation of Sleep Deprivation
It has not been well investigated which of the effects of sleep deprivation on the cardiovascular system (heart and blood vessels) are reversible and to what extent. Thus, further studies will be needed to definitively answer this question.
However, ensuring sufficient sleep duration and good sleep quality is recommended to decrease the risk of hypertension.23 Moreover, ensuring sufficient quantity and quality of sleep is important for overall health.
There are a number of different causes for sleep deprivation. They may include lifestyle choices, work obligations, medical conditions, or other sleep disorders.24 If sleep deprivation is caused by a medical or another sleep condition, or if the cause is not known but the symptoms persist, it is very important to seek a consultation with a healthcare professional. Consulting a professional will ensure that an underlying medical condition does not remain undiagnosed or untreated. If sleep deprivation is the result of lifestyle choices or work obligations, some simple strategies may help: 1) Follow a regular sleep schedule to improve your sleep; 2) Set boundaries in your professional and personal life to ensure you have scheduled enough time for sleep; 3) Perform at least moderate physical activity daily but schedule it at least 5–6 hours before going to sleep; 4) Avoid nicotine, alcohol, and caffeine, and do not eat in the hours preceding going to sleep; 5) Create a relaxing bedtime routine; 6) Create a comfortable atmosphere in your bedroom, such as a cool temperature and a quiet and dark environment; 7) Get sunlight exposure during the day to boost the circadian rhythm; 8) Avoid electronic devices before going to sleep.