Publications

a) Purpose of review:

Current knowledge on inflammatory and central pain-modulatory pathways as potential mechanisms linking sleep disturbances with pain, and the role of sex in modulating these pathways will be reviewed.

b) Recent findings: There is increasing evidence that the degree to which inflammatory and pain-modulatory pathways are affected by sleep disturbances are modulated by sex, with recent studies suggesting a greater pro-inflammatory response in males compared to females, and a stronger impairment of pain-inhibitory pathways in females compared to males.  

c) Summary: Knowledge on sex-differential activations of pain pathways by sleep disturbances has important therapeutic implications and may require different mechanism-based therapies to treat pain in both sexes effectively. The development of specific therapeutics that complement traditional sleep-improving therapies has the potential to better mitigate pain associated with sleep disturbances and interrupt the transition to pain chronicity in both sexes.

Insomnia is a highly prevalent condition that predisposes to the development of many chronic pain disorders, with most of them showing a pronounced sexual dimorphism. We investigated whether prolonged exposure to experimental insomnia-like sleep disturbances affect spontaneous pain and pain sensitivity, and whether responses are modulated by sex. Twenty-four healthy participants (50% females, 28.3±5.9 years of age) underwent two 19-day long in-laboratory protocols in randomized order – an experimental sleep disturbance protocol consisting of repeated nights of short and disrupted sleep with intermittent nights of recovery sleep, and a sleep control protocol consisting of nights with an undisturbed 8-hour sleep. Spontaneous pain was assessed through electronic rating scales throughout the daytime and nighttime wake periods of the protocol. Pain sensitivity was assessed through pressure and heat pain threshold measures at every other day of the protocol. With respect to spontaneous pain, females responded with greater pain to sleep disturbances, while males responded with less pain (p<0.05 for condition*sex). With respect to pain sensitivity, females developed greater pressure pain sensitivity, while males develop greater pain sensitivity to heat pain (both p<0.05 for condition*sex) following the exposure to experimental sleep disturbances. Nighttime pain was greater than daytime pain and worsened across successive nights of sleep disturbances (p<0.05 for condition*day*daytime/nighttime), suggesting that pain management strategies may specifically target nighttime pain. In conclusion, sex strongly affects spontaneous pain and pain sensitivity responses to sleep disturbances, and these sex-differential responses may be one factor contributing to the overrepresentation of females in most chronic pain disorders.

Trial Registration: ClinicalTrials.gov NCT03377543.

Background: Sleep deficiencies, such as manifested in short sleep duration or insomnia symptoms, are known to increase the risk for multiple disease conditions involving immunopathology. Inflammation is hypothesized to be a mechanism through which deficient sleep acts as a risk factor for these conditions. Thus, one potential way to mitigate negative health consequences associated with deficient sleep is to target inflammation. Few interventional sleep studies investigated whether improving sleep affects inflammatory processes, but results suggest that complementary approaches may be necessary to target inflammation associated with sleep deficiencies. We investigated whether targeting inflammation through low-dose acetylsalicylic acid (ASA, i.e., aspirin) is able to blunt the inflammatory response to experimental sleep restriction.

Methods: 46 healthy participants (19F/27M, age range 19-63 years) were studied in a double-blind randomized placebo-controlled crossover trial with three protocols each consisting of a 14-day at-home monitoring phase followed by an 11-day (10-night) in-laboratory stay (sleep restriction/ASA, sleep restriction/placebo, control sleep/placebo). In the sleep restriction/ASA condition, participants took low-dose ASA (81 mg/day) daily in the evening (22:00) during the at-home phase and the subsequent in-laboratory stay. In the sleep restriction/placebo and control sleep/placebo conditions, participants took placebo daily. Each in-laboratory stay started with 2 nights with a sleep opportunity of 8 h/night (23:00-07:00) for adaptation and baseline measurements. Under the two sleep restriction conditions, participants were exposed to 5 nights of sleep restricted to a sleep opportunity of 4 h/night (03:00-07:00) followed by 3 nights of recovery sleep with a sleep opportunity of 8 h/night. Under the control sleep condition, participants had a sleep opportunity of 8 h/night throughout the in-laboratory stay. During each in-laboratory stay, participants had 3 days of intensive monitoring (at baseline, 5^th day of sleep restriction/control sleep, and 2^nd day of recovery sleep). Target variables, including actigraphy-estimated measures of sleep, immune cell function, and C-reactive protein (CRP), were analyzed using generalized linear mixed models.

Results: Low-dose ASA administration reduced the interleukin (IL)-6 expression in LPS-stimulated monocytes (p<.05 for condition*day) and reduced serum CRP levels (p<.01 for condition) after 5 nights of sleep restriction compared to placebo administration in the sleep restriction condition. Low-dose ASA also reduced the amount of cyclooxygenase (COX)-1/COX-2 double positive cells among LPS-stimulated monocytes after 2 nights of recovery sleep following 5 nights of sleep restriction compared to placebo (p<.05 for condition). Low-dose ASA further decreased wake after sleep onset (WASO) and increased sleep efficiency (SE) during the first 2 nights of recovery sleep (p<.001 for condition and condition*day). Baseline comparisons revealed no differences between conditions for all of the investigated variables (p>.05 for condition).

Conclusion: This study shows that inflammatory responses to sleep restriction can be reduced by preemptive administration of low-dose ASA. This finding may open new therapeutic approaches to prevent or control inflammation and its consequences in those experiencing sleep deficiencies.

Trial Registration: ClinicalTrials.gov NCT03377543.

STUDY OBJECTIVES: There is strong evidence that sleep disturbances are an independent risk factor for the development of chronic pain conditions. The mechanisms underlying this association, however, are still not well understood. We examined the effect of experimental sleep disturbances (ESDs) on three pathways involved in pain initiation/resolution: (1) the central pain-inhibitory pathway, (2) the cyclooxygenase (COX) pathway, and (3) the endocannabinoid (eCB) pathway.

METHODS: Twenty-four healthy participants (50% females) underwent two 19-day long in-laboratory protocols in randomized order: (1) an ESD protocol consisting of repeated nights of short and disrupted sleep with intermittent recovery sleep; and (2) a sleep control protocol consisting of nights with an 8-hour sleep opportunity. Pain inhibition (conditioned pain modulation, habituation to repeated pain), COX-2 expression at monocyte level (lipopolysaccharide [LPS]-stimulated and spontaneous), and eCBs (arachidonoylethanolamine, 2-arachidonoylglycerol, docosahexaenoylethanolamide [DHEA], eicosapentaenoylethanolamide, docosatetraenoylethanolamide) were measured every other day throughout the protocol.

RESULTS: The central pain-inhibitory pathway was compromised by sleep disturbances in females, but not in males (p < 0.05 condition × sex effect). The COX-2 pathway (LPS-stimulated) was activated by sleep disturbances (p < 0.05 condition effect), and this effect was exclusively driven by males (p < 0.05 condition × sex effect). With respect to the eCB pathway, DHEA was higher (p < 0.05 condition effect) in the sleep disturbance compared to the control condition, without sex-differential effects on any eCBs.

CONCLUSIONS: These findings suggest that central pain-inhibitory and COX mechanisms through which sleep disturbances may contribute to chronic pain risk are sex specific, implicating the need for sex-differential therapeutic targets to effectively reduce chronic pain associated with sleep disturbances in both sexes.

CLINICAL TRIALS REGISTRATION: NCT02484742: Pain Sensitization and Habituation in a Model of Experimentally-induced Insomnia Symptoms. https://clinicaltrials.gov/ct2/show/NCT02484742.

BACKGROUND: Sleep disturbances, as manifested in insomnia symptoms of difficulties falling asleep or frequent nighttime awakenings, are a strong risk factor for a diverse range of diseases involving immunopathology. Low-grade systemic inflammation has been frequently found associated with sleep disturbances and may mechanistically contribute to increased disease risk. Effects of sleep disturbances on inflammation have been observed to be long lasting and remain after recovery sleep has been obtained, suggesting that sleep disturbances may not only affect inflammatory mediators, but also the so-called specialized pro-resolving mediators (SPMs) that actively resolve inflammation. The goal of this investigation was to test for the first time whether the omega-3 fatty acid-derived D- (RvD) and E-series (RvE) resolvins are impacted by prolonged experimental sleep disturbance (ESD).

METHODS: Twenty-four healthy participants (12 F, age 20-42 years) underwent two 19-day in-hospital protocols (ESD/control), separated by > 2 months. The ESD protocol consisted of repeated nights of short and disrupted sleep with intermittent nights of undisturbed sleep, followed by three nights of recovery sleep at the end of the protocol. Under the control sleep condition, participants had an undisturbed sleep opportunity of 8 h/night throughout the protocol. The D- and E-series resolvins were measured in plasma using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

RESULTS: The precursor of the D-series resolvins, 17-HDHA, was downregulated in the ESD compared to the control sleep condition (p <.001 for condition), and this effect remained after the third night of recovery sleep has been obtained. This effect was also observed for the resolvins RvD3, RvD4, and RvD5 (p <.001 for condition), while RvD1 was higher in the ESD compared to the control sleep condition (p <.01 for condition) and RvD2 showed a mixed effect of a decrease during disturbed sleep followed by an increase during recovery sleep in the ESD condition (p <.001 for condition*day interaction). The precursor of E-series resolvins, 18-HEPE, was downregulated in the ESD compared to the control sleep condition (p <.01 for condition) and remained low after recovery sleep has been obtained. This effect of downregulation was also observed for RvE2 (p <.01 for condition), while there was no effect for RvE1 (p >.05 for condition or condition*day interaction). Sex-differential effects were found for two of the D-series resolvins, i.e., RvD2 and RvD4.

CONCLUSION: This first investigation on the effects of experimental sleep disturbance on inflammatory resolution processes shows that SPMs, particularly resolvins of the D-series, are profoundly downregulated by sleep disturbances and remain downregulated after recovery sleep has been obtained, suggesting a longer lasting impact of sleep disturbances on these mediators. These findings also suggest that sleep disturbances contribute to the development and progression of a wide range of diseases characterized by immunopathology by interfering with processes that actively resolve inflammation. Pharmacological interventions aimed at promoting inflammatory resolution physiology may help to prevent future disease risk as a common consequence of sleep disturbances.

TRIAL REGISTRATION: ClinicalTrials.gov NCT02484742.

Epidemiological studies have repeatedly shown that insomnia disorder or symptoms of insomnia are prospectively associated with increased risks for developing a diverse range of medical conditions affecting multiple biological systems. Specifically, insomnia-related disease risks include cardiovascular diseases (Javaheri and Redline, 2017), metabolic diseases, such as type 2 diabetes and metabolic syndrome (Anothaisintawee et al., 2016; Lin et al., 2016), Alzheimer’s disease (Shi et al., 2018), pain (Afolalu et al., 2018), autoimmune diseases (Kok et al., 2016), and cancer (Shi et al., 2020). Insomnia is associated with changes in the regulation of immune, neuroendocrine, and metabolic systems among other physiological consequences, which are thought to mechanistically link insomnia with increased risk for these diseases. The exact pathophysiological mechanisms through which such physiological changes promote disease risk are now intensively studied for some conditions, such as type 2diabetes(Cedernaes et al., 2015; Depner et al., 2014), but less so for others, such as pain or autoimmune conditions (Haack et al., 2020; Zielinski et al., 2019). There is some evidence, however, suggesting that the physiological consequences are most pronounced in insomnia with objective short sleep duration, generally defined as less than 6 h of sleep per night (Vgontzas et al., 2013). For example, the risks of low-grade systemic inflammation, hypertension, or diabetes are greatest in individuals who have insomnia in combination with short sleep (Fernandez-Mendoza et al., 2017; Vgontzas et al., 2013), suggesting that this insomnia phenotype is a biologically more severe form of the disorder.
Fig. 1 depicts immune, neuroendocrine, and metabolic consequences associated with insomnia disorder and their potential role in mediating increased disease risk, which will be reviewed in the following. Autonomic consequences of insomnia (e.g., blood pressure, heart rate variability) will not be discussed here; the interested reader is referred to recent comprehensive reviews (Grimaldi et al., 2019; Nano et al., 2017).

Despite the growing research base examining the benefits and physiological mechanisms of slow-paced breathing (SPB), mindfulness (M), and their combination (as yogic breathing, SPB + M), no studies have directly compared these in a "dismantling" framework. To address this gap, we conducted a fully remote three-armed feasibility study with wearable devices and video-based laboratory visits. Eighteen healthy participants (age 18-30 years, 12 female) were randomized to one of three 8-week interventions: slow-paced breathing (SPB, N = 5), mindfulness (M, N = 6), or yogic breathing (SPB + M, N = 7). The participants began a 24-h heart rate recording with a chest-worn device prior to the first virtual laboratory visit, consisting of a 60-min intervention-specific training with guided practice and experimental stress induction using a Stroop test. The participants were then instructed to repeat their assigned intervention practice daily with a guided audio, while concurrently recording their heart rate data and completing a detailed practice log. The feasibility was determined using the rates of overall study completion (100%), daily practice adherence (73%), and the rate of fully analyzable data from virtual laboratory visits (92%). These results demonstrate feasibility for conducting larger trial studies with a similar fully remote framework, enhancing the ecological validity and sample size that could be possible with such research designs.

STUDY OBJECTIVE: To evaluate how nocturnal timing of sleep restriction affects vigilant attention and mood in healthy controls with normal sleep-wake patterns.

METHODS: A convenience sample from two controlled sleep restriction protocols were used to investigate the difference between 4 hours of sleep early in the night, versus 4 hours late in the night. Volunteers stayed in a hospital setting and were randomized to one of the three conditions: a control (8 hours of sleep each night), an early short sleep (ESS, 2300-0300 hours), and a late short sleep (LSS, 0300-0700 hours). Participants were evaluated with psychomotor vigilance task (PVT) and mood ratings via visual analog scales.

RESULTS: Short sleep conditions led to greater performance decrements than control on PVT. LSS performance impairments were greater than control (lapses, p = 0.011; median RT, p = 0.029; fastest 10%, p = 0.038; reciprocal RT, p = 0.014; and reciprocal 10%, p = 0.005), but had higher positive mood ratings (p = 0.005). LSS also had higher positive mood ratings compared with ESS (p < 0.001).

CONCLUSIONS: The data underscore the negative mood impact of waking at an adverse circadian phase, for healthy controls. In addition, the paradoxical relationship between mood and performance seen in LSS raises concerns that staying up late and waking at the usual rise time may be rewarding in terms of mood, but nonetheless have performance consequences that may not be fully recognized.

CHAPTER HIGHLIGHTS

• That sleep is altered during sickness has been known for millennia. Yet, systematic and controlled studies aimed at elucidating the extent to which sleep is altered in response to immune challenge have only been conducted during the last 30 years.

• Substances historically viewed as components of the innate immune system are now known to be involved in the regulation or modulation of physiological sleep-wake behavior, in the absence of immune challenge. Changes in sleep during immune challenge are actively driven and result from amplification of these physiological mechanisms.
   
• Although the precise changes in sleep-wake behavior depend on the pathogen, route of infection, timing of infection, host species, and other factors, altered sleep during immune challenge is generally characterized by periods of increased non-rapid eye movements (NREM) sleep, increased delta power during NREM sleep, and suppressed REM sleep. Infection-induced alterations in sleep are often accompanied by fever or hypothermia.
   
• Altered sleep has been studied in humans during pathologies and/or infections with pathogens including HIV/AIDS, rhinovirus (common cold), streptococci, trypanosomes, prions, and sepsis. Laboratory animal models include sepsis, influenza and other viruses (gammaherpes virus, vesicular stomatitis virus, rabies, feline immunodeficiency virus), several bacterial species, trypanosomes, and several prion diseases.
   
• Mechanisms that link sleep to innate immunity involve a biochemical brain network composed of cytokines, chemokines, growth factors, transcription factors, neurotransmitters, enzymes and their receptors.  Each of these substances and receptors is present in neurons, although interactions with glia are critical for host defense responses to immune challenge. Redundancy, feed-forward, and feed-back loops are characteristic of this biochemical network. These attributes provide stability and flexibility to the organismal response to immune challenge.

Sleep disturbances, including disrupted sleep and short sleep duration, are highly prevalent and are prospectively associated with an increased risk for various widespread diseases, including cardiometabolic, neurodegenerative, chronic pain, and autoimmune diseases. Systemic inflammation, which has been observed in populations experiencing sleep disturbances, may mechanistically link disturbed sleep with increased disease risks. To determine whether sleep disturbances are causally responsible for the inflammatory changes reported in population-based studies, we developed a 19-day in-hospital experimental model of prolonged sleep disturbance inducing disrupted and shortened sleep. The model included delayed sleep onset, frequent nighttime awakenings, and advanced sleep offset, interspersed with intermittent nights of undisturbed sleep. This pattern aimed at providing an ecologically highly valid experimental model of the typical sleep disturbances often reported in the general and patient populations. Unexpectedly, the experimental sleep disturbance model reduced several of the assessed proinflammatory markers, namely interleukin(IL)-6 production by monocytes and plasma levels of IL-6 and C-reactive protein (CRP), presumably due to intermittent increases in the counterinflammatory hormone cortisol. Striking sex differences were observed with females presenting a reduction in proinflammatory markers and males showing a predominantly proinflammatory response and reductions of cortisol levels. Our findings indicate that sleep disturbances causally dysregulate inflammatory pathways, with opposing effects in females and males. These results have the potential to advance our mechanistic understanding of the pronounced sexual dimorphism in the many diseases for which sleep disturbances are a risk factor.