A review on the effects of sleep duration on hypertension

Toluwase Adewunmi*

1Tianjin University of Traditional Chinese Medicine, Tianjin, China.

Keywords: Hypertension, Sleep duration, Blood pressure, Sleep

Introducion

Hypertension

Hypertension (HTN), also known as elevated blood pressure (BP), is a chronic medical condition in which the pressure in the arteries is persistently high [1]. It occurs when the small blood vessels in the body are narrowed, this causes extreme pressure against the walls of the vessels, hence, promoting the heart to work harder to maintain the pressure [2]. It is one of the most common diseases affecting humans worldwide and a major risk factor for stroke (cerebrovascular disease) and cardiovascular diseases such as coronary heart disease, heart failure, atrial fibrillation, and end-stage kidney disease [3, 4]. The burden of the morbidity and mortality associated with hypertension is a major public health concern worldwide. Globally, cardiovascular disease accounts for about 17 million deaths per year, of which 45% of deaths are due to heart disease and 51% of deaths are due to stroke.

Sleep

Sleep is becoming recognized as public health importance because its disorders affect millions of people [5]. It is a normal state of the mind and body characterized by a recurrent state of reduced interactions with the environment, fluctuations in the hormone levels and relaxation of musculature [6]. Sleep is a repetitive process and occurs in a repeating period or cycle between the rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. The timing of sleep is controlled by the circadian clock and several structures within the brain are responsible for this. The suprachiasmatic nucleus located in the hypothalamus is the most important organ involved in sleep anatomy. The other structures include the thalamus, pineal gland, basal forebrain and amygdala. Several hormones including γ-aminobutyric acid (GABA), melatonin (from the pineal gland), adenosine, cortisol all play special roles in the sleep-wake cycle [7]. The average sleep duration varies with age and an adult requires about 7-8 hours of sleep per day, although sleep duration decreases with age with the decline in deep sleep beginning around age 36-50. Typical daily sleep duration has been decreasing among adults worldwide with median sleep reducing from 8 hours per night to 7 hours per night and with more than 30% now sleeping less than 7 hours every night [8]. The duration of sleep per night can either be reduced (short sleep) or long and both have been associated with adverse health outcomes and deaths. Normal BP is characterized by a 10% reduction in the mean systolic BP during sleep compared to when awake, this is known as nocturnal blood pressure dipping at night. The absence of this normal dipping is a major indicator of cardiovascular morbidity and mortality [9]. There have been contrasting views on the relationship between duration of sleep and hypertension. This review aims to critically summarize the association between sleep duration (short sleep and long sleep) and HTN.

Physiology of sleep

Two main alternating types of sleep, NREM and REM have been proposed as the sleep architecture in literature. The NREM sleep which predominates sleep is divided into four stages represents a characteristic of variations in the muscle tone, brain wave patterns and eye movements as measured with the use of an electroencephalogram (EEG) [10]. This type of sleep occurs at the beginning of the night whereas the REM sleep duration increases during the last sleep cycles. During sleep, the sympathetic nerve activity to the vasculature decreases as the NREM deepens. From the heart rate variability analysis, the NREM sleep is predominated by the parasympathetic activity with a decline in the sympathetic activity. As a result of this, the BP and the heart rate decreases throughout this stage of sleep which is explained by the normal nocturnal BP dipping pattern. Compared to awakening when the sympathetic activity increases and predominates during the REM sleep as the blood pressure approaches wakefulness levels [9].

Short sleep duration

Individuals who sleep less than 6 hours in a 24-hours period are regarded as short sleepers and are prevalent worldwide. In 2012, the prevalence of short sleepers was 29.2%, 11.3% and 9.8% in the United States, Canada and the United Kingdom respectively [11]. Several systematic reviews have shown that short sleepers have associated morbidity and mortality. The health outcomes include but not limited to HTN, diabetes mellitus, stroke, obesity and other metabolic abnormalities [12-15]. Sleep deprivation has also been linked to a reduction in physical strength, neurocognitive and judgment impairment [14].

Short sleep duration and HTN

Sleep duration has decreased by 1.5 to 2 hours per day over the past years and more than one-third of Americans now sleep ＜ 6 hours [16]. Several studies have found the association between short sleep duration. While some researchers found significant relationships between short sleepers and HTN, many trials found no association between these two. Extended periods of short sleep can lead to elevated BP and vascular and cardiac complications [17].

Gangwisch et al. [17] conducted a longitudinal analysis on the publicly available data of the first national health and nutrition examination survey (NHANES) in epidemiologic follow-up studies. HTN was diagnosed with systolic blood pressure (SBP) readings ＞ 140mmHg and diastolic blood pressure (DBP) ＞ 90mmHg, while sleep duration was self-reported. The study revealed that sleep duration of ≤ 5 hours per night was associated with a significant risk of elevated BP (2.10; 95% CI, 1.58 to 2.79) [17].

Another trial carried out by Gottileb et al. [8] found a relationship between self-reported sleep duration and HTN in a U-shaped association cross-sectional community based prospective study of 5910 subjects aged 40-100 years. HTN was defined as SBP of 140mmHg or more and DBP of 90 mmHg or more. Usual sleep duration above or below the median of 7 to 8 hours per night is associated with the prevalence of HTN, particularly at the extreme of fewer than 6 hours per night [8].

A cross-sectional and longitudinal associations (n = 578) between objectively measured sleep by actigraphy and BP in an ancillary study to the coronary artery risk development in young adults (CARDIA) by Knutson et al. [18]. The study found a significant relationship between short sleep duration and prevalent and incident HTN with a strong correlation observed when sleep duration was subjectively and objectively measured [18].

A systematic review and meta-analysis of clinical trials conducted by Cappuccio et al. [19] showed that short sleep duration was associated with a higher risk of developing or mortality from coronary heart disease (P ＜ 0.0001). Similarly, Meng et al. and Guo et al. [20, 21] found out that short sleepers have increased risk of developing HTN: 1.20 (1.06-1.36) and (RR, 1.02; 95% CI, 0.91-1.14; P = 0.732) respectively in a meta-analysis of prospective cohort studies.

Mechanism of short sleep duration and HTN

It is believed that sleep helps the nervous system to remain healthy and also helps the blood to regulate hormones. Lack of adequate sleep hurts the body's ability to regulate stress hormones, leading to elevated BP. Shorter sleep duration will result in prolong exposures to the sympathetic activity which could lead to arterial and left ventricular remodeling which will gradually reset the cardiovascular system to operate at a higher level [17]. The alterations in the quantity of sleep lead to absence nocturnal dipping of the BP with a resultant increase in the sympathetic nervous system activity which leads to an increase in sympathetic tone [22]. This event is associated with NREM sleep. Lack of sleep also changes the activity of the hypothalamic-pituitary-adrenal axis with short-term sleep duration resulting in an inactive period of cortisol secretion and a sluggish clearance of free cortisol which may result in elevated levels of cortisol and subsequent BP increase [23].

Long sleep duration

Studies have defined long sleep duration as sleepers with the highest quartile of sleep duration ≥ 9 hours. A survey conducted in the United States revealed that approximately 8% of adults reported sleeping for 9 or longer hours [24]. Long sleep duration is prevalent in developed countries particularly in Australia, Finland, Germany, Netherlands, Sweden and the United Kingdom [25]. Long sleepers are prone to increased mortality and morbidity compared to short sleepers.

Long sleep duration and HTN

While the majority of the studies recorded significant relationships between short sleep duration and HTN, the reverse is the case for long sleepers. In a meta-analysis and meta-regression conducted by Jike et al. [25] which examined the dose-response relationship between long sleep duration and health outcomes in 5,134,036 participants from 137 prospective cohort studies, although long sleep was significantly associated with mortality and other incident health outcomes such as diabetes mellitus, stroke, obesity and coronary heart disease. A non-significant relationship was found with incident high blood pressure. Also, the study carried out in the sleep heart health trial also found out that long sleepers (≥ 9 hours) have a significant association with HTN compared with normal (7-8 hours) sleepers [8] (OR 1.30; 95% CI, 1.04-1.62). Similarly, there was a positive association between SBP and long sleep duration than short sleep duration is a cohort study conducted by Nagai et al. [26] with 201 elderly individuals aged (79.9 ± 6.4). It is important to note that this study was conducted in elderly patients. Long sleep ≥ 10 hours duration was associated with increased risk of HTN for all age groups except ＜ 24 years and more than 74 years as discovered in a cross-sectional study which evaluated the relationship between sleep duration and elevated BP conducted by Grandner et al [27].

Two studies [20, 21] found less evidence to support the association between long sleep duration and HTN in a longitudinal study but not cross-sectional studies. Similarly, the systematic review and meta-analysis of prospective studies conducted by Cappucio et al. [19] also found out that long sleeping duration was also associated with a greater risk of coronary heart disease (1.38, 1.15-1.66, P = 0.0005) with no evidence of heterogeneity between the studies.

Discussion

Several types of research have demonstrated that a strong relationship exists between short sleep duration, long sleep duration and risk of HTN. Although, some studies further found out that association exists between short and long sleepers and age, however, this is beyond the scope of this research. Short sleep duration is found to be consistently associated with prevalent and incident elevated BP in the young and middle-aged adults [8, 17], while long sleep duration is found to be associated with HTN in the elderly. Post-menopausal women appear to have a strong relationship between short sleep duration and HTN [28,29], although, some studies refuted this statement. Most of the studies included in this research are cross-sectional, longitudinal and cohort trials. Future studies should consider designing their research as a randomized controlled clinical trial. The proposed mechanism of short sleep duration and HTN has been linked with an increase in sympathetic activity in the NREM sleep. Few studies have focused on the potential mechanisms underlying the association between long sleep duration and cardiovascular risk, although evidence suggests that long sleep is more detrimental than short sleep in terms of increased morbidity and mortality. Similar mechanisms are believed to associate both long and short sleep with HTN as long sleep is linked to increased sleep fragmentation and increased sympathetic nervous system activity [26]. Further studies should examine the pathophysiology of long sleep duration with a detailed association with sympathetic activity. Some limitations in the trials reviewed include the accurate measurement of sleep duration. Whilst some trials obtained the sleep duration by self-report, others used actigraphy or polysomnogram. Although, both methods of measurement showed a significant relationship between sleep duration and HTN. As a result of the morbidities and mortalities associated with short and long sleep duration, it is recommended that adequate sleep (7-8 hours per night) is optimal for adults.