Sympathetic neural responsiveness to sleep deprivation in older adults: sex differences

Abstract

Our laboratory has previously reported that total sleep deprivation (TSD) modifies muscle sympathetic neural activity (MSNA) differently in young men and women. Because postmenopausal women are among the highest risk for hypertension, this study compares MSNA responses with TSD in older men and women. We hypothesized that TSD would alter MSNA in older adults, with greater sympathoexcitation in postmenopausal women. Twenty-seven participants (14 men and 13 women) between the ages of 55 and 75 yr were tested twice, once after 24-h TSD and once after normal sleep (randomized, crossover design). Our primary outcome measure of MSNA (microneurography) was successful across both conditions in 20 participants (10 men and 10 women). Secondary outcome measures included seated blood pressure, heart rate, and fasting plasma testosterone, estradiol, and progesterone. Age (60 ± 1 vs. 61 ± 2 yr) and BMI (27 ± 1 vs. 26 ± 1 kg/m²) were not different between groups. TSD increased systolic blood pressure in both men (124 ± 5 to 130 ± 4 mmHg) and women (107 ± 5 to 116 ± 4 mmHg), but the increases were not different between groups (condition, P = 0.014; condition × sex, P > 0.05). In contrast, TSD elicited divergent MSNA responses in older men and women. Specifically, MSNA burst frequency increased in postmenopausal women (28 ± 3 to 34 ± 3 burst/min), but not older men (38 ± 3 to 35 ± 3 bursts/min; condition × sex, P = 0.032). In conclusion, TSD elicited sympathoexcitation in postmenopausal women but not age-matched men. These findings provide new mechanistic insight into reported links between sleep deprivation and hypertension.

NEW & NOTEWORTHY Epidemiological studies report that sleep deprivation is more strongly associated with hypertension in women than in men. In the present study, 24-h total sleep deprivation (TSD) increased blood pressure in postmenopausal women and age-matched men. In contrast, only women demonstrated increases in muscle sympathetic nerve activity after TSD. The sympathoexcitation observed in postmenopausal women suggests a potential contributing mechanism for epidemiological observations and advances our understanding of the complex relations between sleep, sex, and hypertension.

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INTRODUCTION

Insufficient sleep is a rapidly growing public health concern, and there is accumulating evidence that it contributes to a number of adverse cardiovascular conditions, including hypertension. It has been nearly 13 yr since Gangwisch et al. (16) and Gottlieb et al. (19) analyzed the National Health and Nutrition Examination Survey (NHANES) and Sleep Heart Health studies, respectively, to document a significant cross-sectional relationship between short sleep duration and hypertension. One year after those original studies (16, 19), a larger data set from the UK (Whitehall II study) included sex as a key biological variable and reported that short sleep durations were associated with hypertension in women but not men (5). More recently, Grandner et al. (20) aggregated data across the 2013 Behavioral Risk Factor Surveillance System and 2007–2016 National Health Interview Surveys to establish a data set of more than 700,000 adults and reported that the relationship between short sleep and hypertension was stronger in women than men and that this relationship persists across the lifespan. Taken together, there is compelling epidemiological evidence that insufficient sleep and hypertension are associated and that this relationship is stronger in women compared with men.

Although there are likely several mechanisms that might be contributing to the relationship between sleep deprivation and hypertension, the sympathetic nervous system has been implicated as a potential key contributor. One common experimental approach to examine the putative interactions between sleep deprivation, sympathetic activity, and human hypertension is the 24-h total sleep deprivation (TSD) model. In a prior study conducted in young, healthy adults, Carter et al. (7) reported an acute hypertensive response to 24-h TSD in both men and women. However, peripheral sympathetic tone assessed via microneurography demonstrated that muscle sympathetic nerve activity (MSNA) responded differently to TSD in young men and women. Specifically, MSNA was significantly reduced in young men, a finding consistent with two prior studies (25, 35), whereas MSNA tended to increase in young women (7). These prior findings in young men and women suggest that whereas TSD elicits an acute hypertensive response, the underlying mechanisms (i.e., sympathetic versus alternative) may differ.

The prior findings of Carter et al. (7) were limited to young adults, a population at lower risk for hypertension. In contrast, older adults have a much higher risk of hypertension, with postmenopausal women being the highest risk (17, 40). Indeed, Narkiewicz et al. (34) have shown that MSNA increases much more aggressively for each decade of life in women when compared with men. Accordingly, the present study examined MSNA and blood pressure responses to TSD in postmenopausal women and age-matched men. We hypothesized that TSD would elicit sympathoexcitation and an acute rise in blood pressure in older adults and that these responses would be significantly augmented in postmenopausal women when compared with age-matched men.

METHODS

Participants

Twenty-seven healthy older adults (age range, 55–75 yr; 14 men and 13 women) participated in the study. All participants were nonsmokers, had no history of autonomic dysfunction, cardiovascular disease, asthma, or diabetes, and were not prescribed any cardiovascular or antihypertensive medications. Participants were instructed to abstain from alcohol for 72 h and from exercise and caffeine for 12 h before laboratory testing. All female participants were ≥5 yr postmenopausal and were excluded if they were taking hormone replacement therapy.

All participants were screened for obstructive sleep apnea by a board-certified sleep physician (C. A. Smoot) using the at-home ApneaLink (ResMed, San Diego, CA). Participants with an apnea-hypopnea index (AHI) of ≥30 episodes/h at night were referred for an overnight polysomnography (PSG) at the Portage Health Sleep Center for confirmation of sleep apnea. One male and one female were withdrawn from the study because they were diagnosed with severe sleep apnea (AHI ≥30) after the overnight PSG testing. For three participants (2 women and 1 man), a quality nerve recording site was not obtained, and thus data from those three subjects are not included in final analysis. One male participant was excluded because he revealed after enrollment that he had a high-dose intra-articular corticosteroid injection immediately before one of the laboratory visits for knee pain. Another male participant withdrew from the study due to an unforeseen surgical procedure after the initial laboratory visit. Accordingly, the final reported data set included 10 men and 10 women (Table 1 and 2). Testing procedures were explained to all subjects before written informed consent was obtained and were approved by the Michigan Technological University Institutional Review Board. This study conforms with the guidelines contained within the Declaration of Helsinki.

Table 1.

Baseline characteristics and sex steroids

	Men		Women		P Values
Variable	NS	TSD	NS	TSD	Condition	Sex	Condition × Sex
Age, yr	60 ± 1		61 ± 2			0.488	—
BMI, kg/m2	27 ± 1		26 ± 1			0.856	—
AHI, episodes/h	8 ± 2		8 ± 2			0.953	—
TST, h	6.8 ± 0.3	7.1 ± 0.4	7.6 ± 0.3	7.2 ± 0.4	0.881	0.332	0.129
STAI, arbitrary units
Raw	28 ± 2	29 ± 3	28 ± 2	33 ± 3	0.127	0.423	0.322
Standard	43 ± 2	45 ± 3	45 ± 2	51 ± 3	0.104	0.232	0.290
Percentile	32 ± 8	36 ± 8	38 ± 7	52 ± 8	0.155	0.243	0.447
Estradiol, pg/ml	14 ± 2	14 ± 2	5 ± 2	6 ± 2	0.652	0.005	0.873
Progesterone, ng/ml	0.4 ± 0.1	0.4 ± 0.1	0.2 ± 0.1	0.3 ± 0.1	0.133	0.008	0.525
Testosterone, ng/dl	454 ± 46	390 ± 46	14 ± 44	15 ± 44	0.061	<0.001	0.055

Values are means ± SE; n = 10 men and 10 women. AHI, apnea-hypopnea index, BMI, body mass index; NS, normal sleep; STAI, state-trait anxiety inventory; TSD, total sleep deprivation; TST, total sleep time before each experimental condition. TST, n = 19 (9 men and 10 women); STAI, n = 19 (9 men and 10 women); sex steroids, n = 19 (9 men and 10 women). Repeated-measures analysis of variance (ANOVA) with sex (men vs. women) as the between factor and condition (NS vs. TSD) as the within factor were used to compare TST, STAI, and sex steroids; independent t-tests were used to compare age, BMI, and AHI.

Table 2.

Hemodynamic and neural responses to TSD

	Men		Women		P Values
Variable	NS	TSD	NS	TSD	Condition	Sex	Condition × Sex
Arterial BP, mmHg
Systolic	124 ± 5	130 ± 4	107 ± 5	116 ± 4	0.014	0.025	0.632
Diastolic	76 ± 3	77 ± 2	63 ± 3	64 ± 2	0.587	0.001	0.710
Mean	92 ± 4	94 ± 3	78 ± 4	82 ± 3	0.138	0.003	0.661
Heart rate, beats/min	61 ± 3	58 ± 3	59 ± 3	56 ± 3	0.030	0.687	0.799
MSNA, bursts/min	38 ± 3	35 ± 3	28 ± 3	34 ± 3*	0.323	0.135	0.032
MSNA, bursts/100hb	70 ± 6	65 ± 7	50 ± 6	61 ± 7*	0.476	0.165	0.030
sympBRS, BI/mmHg	−2.5 ± 0.4	−1.7 ± 0.4	−1.9 ± 0.5	−1.5 ± 0.5	0.256	0.308	0.707
cvBRS u-u, ms/mmHg	7 ± 1	11 ± 2	11 ± 2	13 ± 2	0.080	0.122	0.693
cvBRS d-d, ms/mmHg	7 ± 1	9 ± 2	11 ± 2	12 ± 2	0.217	0.062	0.684
Respiration, breaths/min	14 ± 1	13 ± 2	11 ± 1	12 ± 1	0.735	0.313	0.116

Values are means ± SE; n = 10 men and 10 women. BI, burst incidence; BP, blood pressure; cvBRS d-d, spontaneous cardiovagal baroreflex sensitivity for down-down sequences; cvBRS u-u, spontaneous cardiovagal baroreflex sensitivity for up-up sequences; MSNA, muscle sympathetic nerve activity; sympBRS, spontaneous sympathetic baroreflex sensitivity; NS, normal sleep; TSD, total sleep deprivation. sympBRS, n = 13 (7 men and 6 women). Repeated-measures analysis of variance (ANOVA) with sex (men vs. women) as the between factor and condition (NS vs. TSD) as the within factor were used to compare reported variables.

^*P < 0.05 compared with corresponding NS.

Experimental Design

Each participant was tested twice: once after 24-h TSD in the laboratory and once after normal sleep (NS) at their homes. Trial order (TSD vs. NS) was randomized, and tests were performed ∼1 mo apart to ensure that participants in this study were tested at similar intervals as the young adults in our previously published work (7). Wrist actigraphy (Actiwatch Spectrum Pro; Respironics, Bend, OR) and sleep diaries were used to monitor sleep duration for a minimum of four consecutive nights immediately preceding each trial. All wrist actigraphy data were analyzed by a board-certified sleep physician (B. Mokhlesi). The actigraphy/sleep diary data from the three nights preceding each autonomic test demonstrate that participants were getting adequate and similar sleep before the NS and TSD trials (Table 1).

During the TSD trial, participants were contacted at 7 AM the morning before the scheduled sleep deprivation night. They were instructed to remain awake (i.e., no naps) and report to the laboratory at 11 PM, where two research assistants supervised them throughout the remainder of the night to ensure they remained awake (continuous visual observation and periodic auditory confirmations). Participants did not eat after 11 PM to ensure a minimum of 8 h of fasting before the start of laboratory testing. Participants were provided a controlled light breakfast (i.e., water and granola bar) during each testing day (NS and TSD) after the resting seated blood pressure measurements and blood draw were completed by a registered nurse.

On each day of testing (NS and TSD), three seated resting blood pressures were taken using an automated sphygmomanometer (Omron HEM-907XL; Omron Health Care) at 7:30 AM after 5 min of quiet rest. After the blood pressure recordings, state anxiety was measured using the State-Trait Anxiety Inventory (STAI) questionnaire for adults, as previously described (7), and fasting venous blood samples were then obtained to determine levels of estradiol, progesterone, and testosterone. STAI is reported as raw scores as well as standard and percentile scores based on age and sex (Table 1). Participants were then situated in the supine position on a cushioned laboratory table for hemodynamic and microneurographic instrumentation. After instrumentation (including microneurography), all subjects were provided a minimum of 10 min quiet, nonrecorded rest to confirm hemodynamic and neural stability. Following this nonrecorded rest, a 5-min supine baseline was recorded, which was used for all subsequent hemodynamic and neural analyses. Subjects were continuously monitored (visual monitoring with nonverbal feedback from subjects approximately once/min) by study investigators to ensure that they did not fall asleep during the experiment.

Measurements

Microneurography.

Multifiber recordings of muscle sympathetic nerve activity (MSNA) were made by inserting a tungsten microelectrode (Frederick Haer, Bowdoin, ME) into the peroneal nerve of the right leg. A reference electrode was inserted subcutaneously 2–3 cm from the microneurography electrode. Both electrodes were connected to a differential preamplifier and then to an amplifier (total gain of 80,000), where the nerve signal was band-pass filtered (700–2,000 Hz) and integrated (time constant, 0.1 s) to obtain a mean voltage display of nerve activity. Acceptable recordings of MSNA were defined by spontaneous, pulse-synchronous bursts that increased during end-expiratory apnea and remained unchanged during auditory stimulation or stroking of the skin.

Blood pressure and heart rate.

Arterial blood pressures were obtained in the seated position (described previously). Additionally, beat-to-beat arterial blood pressure was recorded continuously throughout the baseline using the Finometer (Finapres Medical Systems, Amsterdam, The Netherlands). Arterial blood pressures are expressed as systolic arterial blood pressure (SAP), diastolic arterial blood pressure (DAP), and mean arterial blood pressure (MAP). Heart rate (HR) was recorded continuously via a three-lead electrocardiogram, and respiratory rate was continuously measured using a pneumobelt.

Sex steroids.

Morning levels of estradiol, progesterone, and testosterone were tested in all participants following an overnight fast. Blood samples were collected in a serum separator tube (SST) without anticoagulant. Following blood clotting, the samples were centrifuged at 3,500 rpm for 10 min. Circulating sex steroids were measured using an electrochemiluminescence immunoassay (ECLIA) method on Roche cobas e 411. The cobas ECLIAs (Roche Diagnostics, Mannheim, Germany) are based on the competition of analyte in sample with a ruthenium-labeled analog. A voltage is applied, and an electrochemiluminescence signal is detected. All blood samples were centrifuged the same day of the experiment, and serums were frozen until being run in batch (with multiple batches over the course of the study). Testing was performed according to the manufacturer’s instructions in the chemistry laboratory at Aspirus Hospital.

Data Analysis

Muscle sympathetic nerve activity.

Data were imported and analyzed in the WinCPRS software program (Absolute Aliens, Turku, Finland). R-waves were detected and marked in the time series. Muscle sympathetic nerve bursts were automatically detected on the basis of amplitude using a signal-to-noise ratio of 3:1 within a 0.5-s search window centered on a 1.3-s expected burst peak latency from the previous R-wave. Potential bursts were displayed and edited by one trained investigator (J. R. Carter), who was blinded to the condition and sex during analysis to avoid bias. MSNA was expressed as burst frequency (in bursts/min) and burst incidence (in bursts/100 heart beats). Resting MSNA was successfully recorded during both NS and TSD conditions in 20 subjects (10 men and 10 women). Figure 1 depicts representative neurograms for one male and one female subject.

Fig. 1.

Representative neurogram of 1 male and 1 female participant following a normal night of sleep (NS) and 24 h of total sleep deprivation (TSD). Whereas women demonstrated a sympathoexcitatory response to TSD, men did not.

Baroreflex sensitivity analyses.

Cardiovagal and sympathetic baroreflex sensitivities (BRS) were determined using spontaneous approaches. Spontaneous cardiovagal BRS was determined from beat-to-beat changes in R-R interval (RRI) and SAP. This sequence method was originally reported by Bertinieri et al. (3) and modified by Blaber et al. (4). Briefly, three or more beats of progressive changes of SAP and corresponding changes of RRI (lag 1) were identified as baroreflex sequences. Both up-up (progressive increases of SAP, followed by a lengthening of the RRI) and down-down sequences (progressive decreases of SAP with a subsequent shortening of the RRI) were recorded. Minimum criteria for accepting a sequence were set at 1 mmHg for SAP and 4 ms for RRI. The slope of the relationship between RRI and SAP was determined by linear regression analysis for each sequence. A minimum r value of 0.7 was used as the criteria for accepting sequences. Up-up or down-down sequences within the 5-min baseline were averaged for each subject.

Sympathetic BRS was determined using the spontaneous DAP-MSNA slope method, which examines relations between spontaneous fluctuations in DAP and MSNA at rest (15, 38, 39). Detailed descriptions of this analysis are presented elsewhere (9, 26). Briefly, DAPs for each cardiac cycle were grouped into 3-mmHg intervals (bins) during baseline. Burst incidence for each DAP bin was calculated and plotted against the corresponding DAP. The slopes of these relationships were evaluated using linear regression analysis. Linear regression analyses were weighted for the number of cardiac cycles within each DAP bin. Inclusion criteria for the sympathetic BRS included negative slopes with a minimum r value of 0.40, which is consistent with our prior study examining TSD in young men and women (7). Accordingly, three males and four females were excluded from the sympathetic BRS slope analysis; however, the mean coefficient values were ≥0.7 for the remaining seven men and six women.

Statistical Analysis

All data were analyzed statistically using commercial software (SPSS 25.0; IBM SPSS, Armonk, NY). Independent t-tests were used to compare age and body mass index (BMI) before the NS laboratory session. Repeated-measures analysis of variance (ANOVA) with sex (men vs. women) as the between factor and condition (NS vs. TSD) as the within factor was used to compare hormonal, hemodynamic, and neural measurements. Post hoc analysis using a paired t-test was performed when significant condition × sex interactions were detected. Data are presented as means ± SE.

RESULTS

Table 1 compares participant characteristics across conditions (i.e., NS vs. TSD) and/or sexes (male vs. female). Age, BMI, and AHI were not different between men and women. Total sleep time before the NS and TSD sessions were similar between men and women (sex, P = 0.332), respectively, and across conditions (condition, P = 0.881; condition × sex, P = 0.129). TSD did not elicit significant changes in state anxiety scores, and the results were similar across the groups (condition × sex, P = 0.322). Likewise, levels of estradiol, progesterone, and testosterone were not significantly altered by TSD (condition × sex, P > 0.05 for all).

Figure 2 demonstrates that TSD elicited an acute rise in blood pressure in both groups. Specifically, Table 2 shows that SAP increased after TSD in men and women (condition, P = 0.014), but these increases were not different between sexes (condition × sex, P = 0.632). Table 2 shows that TSD did not alter DAP or MAP. Table 2 demonstrates that HR was significantly lower following TSD in men and women (condition, P = 0.030), but these reductions were not different between sexes (condition × sex, P = 0.799). Finally, TSD did not significantly alter cardiovagal and sympathetic BRS (all condition effects, P > 0.05), and responses were similar between sexes (all condition × sex interactions, P > 0.05).

Fig. 2.

Blood pressure and heart rate (HR) after a normal night of sleep (NS) and after 24 h of total sleep deprivation (TSD) in men and women. TSD increased systolic arterial blood pressure (SAP) in both men and women. Diastolic arterial blood pressure (DAP) and mean arterial blood pressure (MAP) were not altered in men or women. TSD decreased HR in both men and women. Repeated-measures analysis of variance (ANOVA) with sex (men vs. women) as the between factor and condition (NS vs. TSD) as the within factor were used to compare blood pressure and heart rate. *P < 0.05 within groups (men or women).

Figure 3 shows the differential neural response of MSNA between men and women when depicted as either burst frequency (condition × sex, P = 0.032) or burst incidence (condition × sex, P = 0.030). Post hoc analysis revealed that TSD significantly increased MSNA burst frequency in women (28 ± 3 vs. 34 ± 3 bursts/min, P = 0.035) but did not alter MSNA burst frequency in men (38 ± 3 vs. 35 ± 3 bursts/min, P = 0.145). Likewise, TSD elicited a significant sympathoexcitatory response in women (50 ± 6 vs. 61 ± 7 bursts/100 heart beats, P = 0.045), when quantified as burst incidence, but not men (70 ± 6 vs, 65 ± 7 bursts/100 heart beats, P = 0.100).

Fig. 3.

Muscle sympathetic nerve activity (MSNA) after a normal night of sleep (NS) and after 24 h of total sleep deprivation (TSD) in men and women. TSD MSNA, when quantified as burst frequency and burst incidence, elicited divergent responses between sexes. TSD increased MSNA burst frequency and burst incidence in women but did not alter MSNA in men. Repeated-measures analysis of variance (ANOVA) with sex (men vs. women) as the between factor and condition (NS vs. TSD) as the within factor were used to compare MSNA. *P < 0.05 within groups.

DISCUSSION

The current study examined the influence of TSD on neural cardiovascular control in older men and women. Similar to prior work conducted in young healthy men and women (7, 25, 35), TSD elicited an acute rise in blood pressure in healthy postmenopausal women and age-matched healthy men. Likewise, there was a significant sex × condition interaction for our primary outcome variable of MSNA, which was consistent with prior work comparing young men and women (7). However, there are subtle, yet important differences between the present study and the prior study in young adults (7). Specifically, post hoc analysis in the study with young adults demonstrated that TSD elicited significant sympathoinhibition in young men but not women (7). In contrast, the post hoc analysis of the present study reveals that TSD elicits significant sympathoexcitation in postmenopausal women, with no significant difference in older men. Together, these studies suggest that the mechanistic differences observed in young adults continue throughout the lifespan, with a stronger predominance toward a TSD-induced sympathoexcitation in women with aging. Taken together, the findings of the present study significantly advance our knowledge regarding the impact of sleep deprivation and sympathetic neural control and suggest sympathoexcitation as a potential mechanism that might contribute to postmenopausal hypertension.

Narkiewicz et al. (34) highlighted a relationship between MSNA and age that is clearly impacted by sex. Specifically, they show that when compared with age-matched men, women tend to exhibit 1) lower baseline MSNA early in their adult lifespan (20–39 yr), 2) roughly similar levels of MSNA around midlife (40–49 yr), and 3) significantly elevated levels of resting MSNA in later life (>60 yr). In fact, baseline MSNA increases threefold more over each decade of life in women when compared with men (34). This increase of sympathetic tone is believed to be a contributor to the dysregulation of blood pressure in older women following menopause. Prior studies have documented complex relations between cardiac output, MSNA, and peripheral blood flow that are strongly influenced by age and sex (11, 23). Furthermore, Barnes et al. (2) reported that α-adrenergic vasoconstriction is offset by β-adrenergic vasodilation in young women, a finding that may help explain why women appear to be “cardioprotected” early in life before an aggressive increase of cardiovascular risk during postmenopausal years. As such, conditions that elicit augmented MSNA in older adults, and particularly women, are clinically relevant.

A limitation of our prior TSD study was the fact that it was conducted in young, healthy individuals (7). Given the increased risk for the development of hypertension with age (29), and particularly in postmenopausal women (17), the present study in older adults was important and timely. Consistent with our hypothesis and the multiple epidemiological studies reporting a stronger relationship between sleep deprivation and hypertension in women (5, 20), a congruent line of research has emerged from the two studies performed in young (7) and older (present study) adults. Specifically, TSD elicits divergent MSNA responses in men and women, with sympathetic predominance in women that gets stronger with age. When coupled with the proposed impact of age-associated increases in sympathetic tone on blood pressure, the present study suggests sleep as a potentially important and cost-effective treatment to postmenopausal hypertension.

Although the present study focused on the impact of acute sleep deprivation on sympathetic activity, other sleep disorder conditions such as obstructive sleep apnea (OSA) and insomnia also contribute excessive sympathoexcitation. OSA itself causes robust increases of MSNA during sleep, and this sympathoexcitatory state appears to carry over during wakefulness (6, 37). Continuous positive airway pressure, the gold-standard treatment for OSA, decreases daytime resting MSNA in patients with OSA (33). Despite well-characterized differences in MSNA between men and women, it remains unclear whether the relationship between OSA and MSNA differs between sexes.

Resting MSNA was recently compared between subjects with insomnia and good-sleeper controls (8). Although there were no group differences for resting MSNA, subjects with insomnia demonstrated a significantly reduced sympathetic BRS and heightened MSNA reactivity to cold pressor test when compared with controls (8). Although the study population included both men and women, it was not powered to assess potential sex differences. Using less invasive heart rate variability assessments of autonomic activity, de Zambotti et al. (13) reported that nocturnal hyperarousal is present with menopause transition insomnia. This was characterized by reduced high-frequency power of HRV in rapid eye movement (REM) and non-REM sleep, an indication of vagal withdrawal from the heart, in menopause transition insomnia women tested in both early follicular and midluteal phases of their menstrual cycle. Additionally, menopause transition insomnia is shown to affect nocturnal blood pressure dipping profiles (14). This is of clinical relevance given that the majority of cardiovascular events occur during the early morning hour just before and after waking. So although evidence to suggest that insomnia is associated with sympathetic excitation and/or vagal withdrawal is accumulating, more research is warranted, and the impact of sex needs to be carefully considered.

If sleep deprivation does indeed impact older men and women differently, other menopausal transitions such as natural versus surgical menopause require more attention. This is particularly relevant given the accelerated increase in MSNA in women following menopause, which contributes to blood pressure regulation (2) and potentially to hypertension (24). Moreover, early-onset menopause and a shorter reproductive lifespan are associated with an increased cardiovascular risk later with aging (28). Finally, women who naturally transition to menopause but report vasomotor symptoms like hot flashes can be at increased risk for cardiovascular diseases (32). This is consistent with the notion that hot flashes may be a symptom of neurovascular dysfunction, rather than simply menopause (30). However, it should be noted that many women who struggle with vasomotor symptoms begin hormone replacement therapy, which can decrease MSNA activity (10, 31). In summary, there is a need to further explore sympathetic activity during various menopausal transitionary conditions as well as the impact of sleep on these undefined relationships.

Serum testosterone levels consistently decrease following TSD in both the rodents and humans (1, 12, 18). However, these prior studies have focused on young, healthy populations. In the present study, testosterone tended to decrease in older men, but this did not quite reach statistical significance (P = 0.06). However, our study was statistically powered for the primary outcome variable of MSNA, and thus it is plausible that we would have reached statistical significance with additional subjects. Also, it was curious that our sample of older men had a significantly higher concentration of plasma estradiol compared with age-matched postmenopausal women, especially given testosterone decreases with age (36). However, it is important to remember that aromatase aids in the conversion of free testosterone to estradiol and that this progressively increases with age (27). This is most prevalent in subcutaneous adipose tissue compared with visceral fat, which is applicable to the present study given the higher percentage of fat mass in older adults (27). It should also be noted that sex hormone-binding globulin increases with age and regulates the balance of free and bound testosterone in the blood (21), thus allowing for a more substantial contribution of aromatase to the conversion of testosterone to estradiol. These factors help explain the higher levels of estradiol exhibited in the older men when compared with the postmenopausal women.

We acknowledge the limitations of the TSD experimental approach. Specifically, although TSD represents a cost-effective intervention to study sleep deprivation, it is more common for humans to be exposed to repeated nights of short sleep (i.e., partial sleep restriction). Indeed, Haack et al. (22) have shown that sleep extension of <1 h in subjects with habitually short sleep duration can result in clinically relevant reductions of blood pressure, but that prior study did not aim to assess MSNA or the impact of sex. Whereas the use of sleep restriction and/or sleep extension protocols represent logical next steps to the current research, we maintain that the sex differences reported in both the present study and our prior work in young adults (7) are scientifically rigorous and an important foundational step to better understanding the complex relations between sleep, sympathetic activity, and sex.

In summary, epidemiological studies have converged to demonstrate that the associations between sleep deprivation and hypertension are stronger for women when compared with men (5, 20). Such associations do not demonstrate cause and effect, and there is a need for further experimental studies to better understand this relationship, particularly in postmenopausal women who are at heightened risk for hypertension and other cardiovascular diseases. Our study demonstrates that whereas TSD elicits an acute increase of blood pressure in both men and women, sympathoexcitation is only observed in postmenopausal women. This sex difference is consistent with prior work conducted in young adults, but with a more robust sympathoexcitatory profile (7). Whereas TSD is a valid experimental approach, future work should examine the relations between sleep, sex, and sympathetic activity in experimental models with greater generalizability and stronger ecological validity, such as sleep restriction or sleep extension.

GRANTS

This study was supported by National Heart, Lung, and Blood Institute Grant HL-122919 and the Portage Health Foundation.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.R.C. and C.A.S. conceived and designed research; J.R.C., I.T.F., I.M.G., and C.E.S. performed experiments; J.R.C., I.T.F., I.M.G., C.E.S., B.M., and C.A.S. analyzed data; J.R.C., I.T.F., I.M.G., C.E.S., B.M., and C.A.S. interpreted results of experiments; I.M.G. prepared figures; J.R.C. drafted manuscript; J.R.C., I.T.F., I.M.G., C.E.S., B.M., and C.A.S. edited and revised manuscript; J.R.C., I.T.F., I.M.G., C.E.S., B.M., and C.A.S. approved final version of manuscript.