Relationship Between Sleep and Hypertension: A Narrative Review

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Hypertension or high arterial blood pressure increases the risk of cardiovascularassociated morbidity and mortality burden worldwide 1 .Hypertension is the cause of about 7.6 million deaths per year across the world.About 47% of atherosclerotic arterial diseases is caused by chronically elevated blood pressure 2 .As a result of issues associated to hypertension, men lose 9.31 years of their lifetime on average, and women lose 9.46 years 3 .There are several studies that prove the consistent positive association between hypertension and cardiovascular diseases 1,4,5 .

Causes of Hypertension
There are several causes of increased blood pressure, such as unhealthy lifestyle, diet, idleness, sleep, smoking and alcohol consumption.Willis-Ekbom disease or restless leg syndrome, which is incontrollable movement of the leg due to uncomfortable sensations, is also known to increase the blood pressure 6 .
One of the major factors responsible for constant elevation of arterial pressure is trouble sleeping.Lack of sleep, inadequate sleep and poor quality of sleep results in high blood pressure which in turn causes a greater likelihood of cardiac diseases 7 .In today's fast paced world, the duration of sleep achieved by an individual especially of the working age group is progressively reducing, along with a compromise in its quality owing to overwhelming stressful conditions in life.Statistical analysis show that around two-thirds of the American population sleep for less than 7 hours on a day-to-day basis 8 .

Mechanism of elevated blood pressure due to short sleep
The mechanism involved as demonstrated by various studies is that shorter sleep durations can interfere with the normal circadian or sleep wake cycle and autonomic balance control 9 .Parasympathetic nervous system prevails during night-time whereas sympathetic stimulation takes over during daytime.The low blood pressure seen at night is known as ''nocturnal dipping''.
Reduced nocturnal dipping is a risk factor for hypertension 6 .Alterations in sleep patterns or sleep disruptions cause an increase in the activity sympathetic nervous system.This increased sympathetic stimulation, hence raises heart rate and blood pressure 10 .
Evidence from the US show that shorter durations of sleep is associated with increased incidence of hypertension 11 .

Risk factors
The risk factors associated with hypertension can be classified into modifiable and non-modifiable factors.Although non-modifiable factors such as age, gender, nationality and genetics aren't under one's control, the modifiable factors can be controlled to a large extent.Changes in one's lifestyle is not only the primary management strategy of hypertension, but is also very crucial in reducing the risk of any future cardiac complications.While lifestyle changes such as dietary modifications, cessation of alcohol intake, smoking and drug use has been widely propagated and practised, sleep amendments are given relatively less importance and thus less commonly adopted.Correction of sleep schedules or patterns is not only one of the preventive measures for hypertension but it has been proven to reduce blood pressure as well.Increasing the level of awareness among the population about the importance of adequate, timely and good quality sleep is vital to achieve an overall decrease in the incidence of life-threatening complications and thus, also achieve a significant rise in the health outcome of the population.
This review also intends to encourage adults mainly of the middle-aged group to follow a healthy sleep schedule and seek medical or psychological help for the management of their sleep problems, which in turn would protect them from the risk of hypertension leading to serious heart diseases.Thus, this review overall aims to emphasize of the association between short sleep and increased risk of hypertension.

MateRials and MetHods
A comprehensive literature search was conducted in 2022 to extract published work on sleep and hypertension.Published articles on the topic were collected by searching various databases that include PubMed, Sciencedirect, Frontiers, Hindawi, ResearchGate and AHA journals in the period of 2006-2022.This time period showed an increase in the prevalence of chronic illnesses in the middle aged population.The keywords used for searching included ''Sleep'', ''Hypertension'', ''Insomnia'' and ''Cardiovascular diseases''.inclusion criteria Both authors participated in the review process.SK and JM reviewed articles on Sleep and hypertension.Studies explaining the association based on age and gender were chosen.The review was completed within a period of four months.During the initial search for articles in the databases, 75 studies were extracted.A total of 49 articles were finally used after adopting the exclusion criteria.The articles used for this narrative review consisted of various study patterns, such as review articles, systematic reviews, meta-analytic studies, crosssectional studies, instrumental evaluation and experimental studies.The information obtained from each publication was abstracted.summary of table 1  49 articles were downloaded from the databases.Out of these, a majority (45) of the articles focus on the effect of shortened and poor sleep quality on blood pressure.Around four articles highlights the association of longer durations of sleep with the development of hypertension.Three articles identified insomnia to be an independent risk factor of hypertension.Five articles focus on the effect of or the relationship between hypertension and cardiovascular diseases.
Although a majority of the articles emphasizes sleep characteristics as a modifiable risk factor of hypertension, a few also highlight the several other modifiable and non-modifiable risk factors.
Majority of the articles highlight the association of short sleep and hypertension among adults, and not children and adolescents.However, one study confirmed the association among young children.In the cross-sectional studies that were conducted, individuals were grouped according to sleep durations they receive, and those falling in the category of sleep received less than 6 to 7 hours belonged to the highest risk group.Two studies reported the relationship to be associated with the population's Body Mass Index, which proves obesity to be a risk factor as well.Around six studies emphasize on this association to be more prevalent in the middle-aged group.Risk of hypertension following longer durations of sleep was also found to be more prevalent among the middle-aged population.Four studies concluded females to be at a greater risk of developing hypertension as a result of shortened sleep compared to males.

Results
A total of 49 studies were included in this review.Assessments of the articles concluded that inadequate sleep, poor sleep quality and lack of sleep or insomnia was seen to be associated with increased risk of hypertension and cardiovascular diseases.
This review proves shortened sleep duration to be one of the risk factors of hypertension.Individuals receiving inadequate sleep (less than 5 hours) were at a higher risk of developing an elevated blood pressure.A majority of the studies used in this review concluded this association to be stronger among females, and those belonging to the middle-aged group.About 10% of the studies used in this narrative review presented an opposite association outcome.These studies reported that not only short sleep, but longer durations of sleep also increased the incidence of hypertension, though there is no sufficient explanation for the association.Insomnia was also seen to be an independent predictor of hypertension.The review also concluded that sufficient sleep at night (6-8 hours) significantly reduces the risk of hypertension.

disCussion
Prior literature presents the various risk factors associated with hypertension.Sleep -related effects on hypertension is one of the modifiable sectors with addition to changes in lifestyle.Recent research has shown that sleep deprivation has direct significant effects on hypertension and coronary artery diseases 12 .A study has shown that during normal night's sleep, blood pressure is lower as compared to daytime during alertness 6 .Through a thorough literature search, the current review concluded a strong association between sleep duration and prevalence of hypertension.It was found that shortened duration of sleep and insomnia were risk factors for the development of hypertension.
There are several studies assessing the association of sleep and hypertension among middle-aged and elderly adults.However there is very few literature available that depicts the association among children and young adults owing to the rarity of occurrence 13 .One study conducted among Chinese children and adolescents showed shortened sleep duration caused an elevated blood pressure 14 .Short sleep duration was shown to be an independent risk factor for hypertension 15 .In a sleep heart health study that was conducted, individuals receiving less than or equal to 6 hours of sleep per week had a greater prevalence of hypertension. 16nalysis of results conducted among participants of the National Health and Nutrition Examination Survey, showed that shorter duration, sleeping troubles and any kind of sleep disorder was associated with increased risk of hypertension 17 .About two-thirds of the American population likely suffer from chronic sleep deprivation, which has shown to be causing an increased prevalence of hypertension 8 .US adults with short sleep reported a greater occurrence of hypertension.However this association was dependent on age and Body Mass Index since blood pressure varies with age and obesity is one of an independent risk factor by itself 18 .
A prospective study conducted among the semi-urban Nigerian population showed that majority of the participants with hypertension reported poor sleep and thus poor quality of life.This study also concluded that poorer quality of sleep worsens hypertension and eventually lead to other heart conditions 19 .
An analysis showed a significant relationship between insufficient sleep and high blood pressure 20 .US citizens belonging to 32-59 years age group who sleep for 5 hours or less reported greater risk of hypertension 11 .A systematic review performed showed congruent results 21 .Nigerian adults having poor sleep also showed a greater risk for hypertension 22 .
A study conducted in France showed that shorter sleep was a major cause of hypertension for adults greater than 40 years of age 23 .
The greater prevalence of this association among the middle aged group can be explained by considering this section of the population to be more exposed to stress from work and family which will eventually lead to an unhealthy lifestyle.However, this association is less prominent among the older age groups because of the physiological increase in blood pressure and other parameters with age.
Numerous studies conducted in China showed congruent results.A cross-sectional study in China concluded that middle aged adults (belonging to the age group of 18-44) sleeping for less than 7 hours per day had a greater risk of hypertension as compared to individuals who got more than 7 hours of sleep 25 .Another study conduct among Chinese males concluded that individuals with poor sleep quality and sleep for less than 6 hours showed greater risk for hypertension 26 .
A study conducted in north-eastern China showed that shorter sleep duration, increased latency of sleep, sleep disruptions and poor sleep quality was related to increased risk of hypertension 27 .Short sleep duration was also associated with increases incidence of atherosclerotic cardiovascular disease (ASCVD) 28 .
A study conducted in Xinjiang, China showed that inadequate and poor sleep was positively associated with hypertension 29 .People having shorter sleep durations showed greater prevalence of hypertension than people with normal sleep durations and patterns 30 .
Another Chinese study concluded similar results 31 .A greater prevalence of hypertension was observed in middle-aged and old adults with fragmented or disrupted sleep patterns 32 .In an analytic study conducted in Korea, it was found that in individuals having sleep of 5 hours or less, there was a significant increase in the pharmacological management for hypertension 33 .In contrast, from a study conducted among non-insomniac elderly subjects, it was found that sleep quality and duration had no association with prevalence of hypertension.It identified Body Mass Index to have a stronger correlation with hypertension 38 .This difference could be attributed to the age-related blood pressure changes that is commonly found among the elderly.
A cross sectional study conducted in Pennsylvania suggested that the risk of hypertension was greater among individuals with less than 5 hours of sleep.However, it was shown to be of a lesser magnitude 34 .
While most of the studies included in the current review highlighted short sleep to cause elevated blood pressure, an analysis showed that excess or longer duration of sleep increased the risk of hypertension 24 .A systematic review that was conducted to analyse the correlation between short or long duration of sleep showed that a U-shaped relationship between the variables was established 49 .
Similarly, meta-analytic study for finding the association between sleep and hypertension showed that, not only shorter durations, but also longer durations of sleep was associated with increased risk of high blood pressure 9 .A study conducted among Chinese adults showed that both shorter and longer duration of sleep can increase the risk of hypertension among the greater than 45 years age group.This study also demonstrated a preventive pattern of napping during the day 35 .Similar results were derived from a study conducted in the US 36 .Another report arrived at similar conclusions 37 .Adequate research data is not available to explain this association.The possible explanation for this could be that excessive sleep elevates blood sugar levels, which in turn can cause a rise in blood pressure.
In contrast, a longitudinal study conducted in China showed only shorter duration of sleep, to have a significant effect on blood pressure 39 .It was found that treatment of sleep problems in middle-aged adults could significantly reduce the development of hypertension 40 .A study conducted among hypertensive patients in Japan showed that reduced sleep was an important risk factor 41 .In a study conducted among 131 patients with Stage 1 hypertension, it was found that poor sleep was a major factor for the development stage 1 hypertension 42 .
Several studies have shown that this association is stronger among women than in men 6,8,42,44 .This can be attributed to the fact that women, especially of the middle aged group, bear the burden of household responsibilities along with work which provides them with minimal amount of sleep and rest.Women tend to be more stressed and anxious due to the overwhelming number of duties they are expected to fulfil.A study conducted among white men and women showed that less than 6 hours of sleep was significantly associated with increased risk of hypertension only in females and not in males.It was also seen to be stronger in pre-menopausal women 44 .
Insomnia that is considered to be chronic showed present with symptoms at least 3 times a week for three months 45 .Patients suffering from chronic insomnia showed a greater association with both stage 1 and stage 2 of hypertension.The study concluded that insomnia should be considered as a predictor during evaluation of hypertension 47 .Elevated blood pressure associated with insomnia can be explained by the increased sympathetic activity as well as increased action of hypothalamic-pituitary system on the adrenal glands, which increases the levels of cortisol 43 .
There is a concurrent existence of insomnia and hypertension 47 .Insomnia has also been identified as a predisposition factor for the development of hypertension 6 .A higher prevalence of hypertension was seen among people suffering from insomnia as compared to the people not suffering from any kind of sleep disruption 8 .
A study conducted among the rural population in China suggested that having 6-8 hours of sleep at night and a one hour nap during daytime shows a significant reduction in the risk of hypertension 48 .

ConClusion
Hypertension is one of the prime chronic condition that commonly prevails among the adults, especially in this new age and time of unhealthy and poor lifestyle.Hypertension is a salient risk factor for the development of heart diseases [4] , and ultimately leads to fatal consequences if turned a blind eye to.According to this review, a significant association between sleep and hypertension was found from various literature sources.Furthermore, the risk of hypertension following poor and inadequate sleep was found to be more prominent among females and individuals belonging to the middle-aged group.Insomnia was also found to be an independent predictor for development of hypertension.Systematic changes to the sleep schedule, disciplining of sleep habits and seeking professional help for sleep-related problems will prove to be pivotal in reducing the population's risk of developing not only hypertension but also other associated chronic illnesses such as diabetes, obesity and depression.
The review recommends the healthcare practitioners to advice disciplined sleep cycle patterns to their patients along with other lifestyle changes, which will aid in the overall augmentation of protection against hypertension.aCknowledgeMent I would like thank Dr. Jayakumary Muttappallymyalil for her guidance and inputs during the editing of the review, which gave it structure and enabled its proper refinement.

Conflict of Interest
There are no conflict of interests in association with the material presented in this paper.

Funding sources
There is no funding source.

table 1 .
Summary of articles included in the narrative review