Sleep is a fundamental physiological process critical for human health and well-being. It plays a vital role in various bodily functions including cellular repair, memory consolidation, metabolic regulation, and emotional stability. Despite its importance, modern lifestyle factors—ranging from work-related stress and digital device overuse to environmental disturbances—have significantly impacted sleep quality for millions globally. Poor sleep quality, characterized by insufficient sleep duration, frequent awakenings, and non-restorative sleep, is increasingly recognized as a multifactorial risk factor for a broad spectrum of psychological and physical health issues.

Numerous epidemiological and experimental studies have demonstrated the bidirectional relationship between sleep and health, with evidence suggesting that sleep disruptions can both result from and contribute to chronic illnesses [1]. The World Health Organization has acknowledged sleep disorders, particularly insomnia, as major contributors to the global burden of disease [2]. In the United States alone, an estimated 70 million individuals suffer from some form of sleep disorder, with insomnia being the most prevalent [3]. Furthermore, the prevalence of poor sleep quality is particularly high among vulnerable populations, including shift workers, older adults, adolescents, and individuals with comorbid psychiatric or physical illnesses [4,5].

Psychologically, the lack or divided sleep is strongly related to the emergence and the progress of mood problems (e.g., depression, anxiety), bipolar disorder, and post-traumatic stress disorder (PTSD) [6]. Sleep disturbance is thought to affect not only emotional processing, but also stress management and cognitive flexibility, which are expressed as irritability, instability of mood and impaired concentration. Imaging studies have associated poor sleep with functional changes in the amygdala, prefrontal cortex, and hippocampus—regions important for emotion regulation and memory formation [7,8]. These modifications can enhance the susceptibility to the development of psychiatric disorders and impede recovery [9].

Bad sleep has an equally dramatic impact on the body. Disturbances in sleep have been associated with enhanced sympathetic nervous system activity, hypercortisolism and impaired glucose metabolism [10]. Short sleep duration impairs insulin senstivity as well as regulation of leptin and ghrelin leading to weight gain and metabolic syndrome [11]. Finally, long-term sleep loss is related to increased blood pressure and endothelial dysfunction and thus may be considered a risk factor for the development of cardiovascular diseases (CVDs: coronary artery disease (CAD), heart failure (HF) and stroke [12, 13].

The immune system is another key domain affected by sleep quality. Numerous studies have documented the impact of poor sleep on immune function, highlighting reduced natural killer (NK) cell activity, increased production of pro-inflammatory cytokines like IL-6 and TNF-α, and impaired vaccine response [14,15]. These immune alterations can compromise the body’s ability to defend against infections and are believed to play a role in the pathogenesis of autoimmune diseases and cancer [16,17]. Moreover, poor sleep has been implicated in neurodegenerative diseases like Alzheimer’s and Parkinson’s, potentially through mechanisms involving oxidative stress and impaired clearance of neurotoxic waste products such as beta-amyloid [18,19].

Sleep is also integral to maintaining cognitive performance. Poor sleep has been shown to impair attention, executive functioning, and working memory, which in turn affects academic achievement and workplace productivity [20]. In adolescents, insufficient sleep has been associated with lower academic performance, increased risk-taking behavior, and greater emotional dysregulation [21]. In older adults, sleep disturbances can accelerate cognitive decline and increase the risk of dementia [22].

Social and environmental determinants are also key in influencing sleep health. Shift work, which suppresses diurnal activity, is known to be a risk factor for cardiovascular disease, cancer, and mental illnesses [23]. Moreover, artificial light at night, which has increasingly become a side effect of widespread digital screen display, may disrupt melatonin synthesis, thus delaying the sleep onset and decreasing sleep quality [24]. Psychological stress, SES, and cultural norms regarding sleep also contribute to differences in sleep health, which are challenging obstacles to overcome when trying to improve sleep hygiene at the level of the population.

Considering the commonality of sleep disturbances and the implications that they have on population health, it is important to find holistic and multisystemic strategies to manage poor sleep quality. This encompasses public health messaging around awareness, workplace-based sleep changes, and clinical interventions such as cognitive behavioral therapy, pharmacotherapy, and sleep-hygiene education.

This systematic review aims to collate evidence on the psychological and physiological effects of low sleep quality, focusing on mechanisms, consequences, and interventions. By systematically synthesizing findings from a wide range of studies, it is hoped that this review will guide future research and public health approaches to improve sleep health at the population level.

Key Health Domains Affected by Poor Sleep Quality

This systematic review adhered to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines to ensure rigorous methodology and reproducibility [25]. Literature searches were performed across major databases including PubMed, Scopus, Web of Science, and PsycINFO, covering publications from January 2000 to February 2024. The search strategy incorporated keywords and MeSH terms such as "sleep quality," "sleep deprivation," "insomnia," "mental health," "cognitive function," "metabolic syndrome," "cardiovascular disease," and "immune response."

Eligibility Criteria: Studies were included if they met the following criteria:

• Published in peer-reviewed journals.
• Included human participants.
• Examined associations between sleep quality and psychological or physical health
• outcomes.
• Employed quantitative methodologies such as cohort, case-control, cross-sectional, or randomized controlled trial designs.
• Published in English.

Exclusion criteria comprised:

• Non-peer-reviewed studies (e.g., dissertations, editorials).
• Studies focusing on pediatric populations below age 10.
• Articles lacking objective or self-reported sleep quality measures.

Study Selection: Two independent reviewers conducted the screening process. Titles and abstracts were initially reviewed for relevance, followed by a full-text review for those meeting the inclusion criteria. Discrepancies were resolved by consensus or consultation with a third reviewer.

Data Extraction and Quality Assessment: Key data extracted included study design, sample size, participant characteristics, sleep assessment tools, and health outcomes measured. The Newcastle-Ottawa Scale (NOS) was used for quality assessment of observational studies, while the Cochrane Risk of Bias tool was applied to randomized controlled trials [26,27]. Only studies with moderate to high-quality ratings were retained.

Data Synthesis: Due to methodological heterogeneity, a narrative synthesis approach was adopted. Studies were grouped based on outcome domains: psychological (e.g., depression, anxiety, cognitive decline), metabolic (e.g., obesity, insulin resistance), cardiovascular (e.g., blood pressure, heart rate variability), and immune (e.g., cytokine levels, vaccine efficacy).

PRISMA Flowchart: The PRISMA flow diagram below illustrates the screening and selection process:

This methodology ensures a comprehensive, unbiased, and evidence-based examination of the relationship between poor sleep quality and psychological and physical health outcomes [28–31].

The included studies consistently demonstrate a multidimensional impact of poor sleep quality across psychological, metabolic, cardiovascular, and immune-related health outcomes.

Psychological Health Outcomes: Sleep disturbances are strongly linked with increased risks of depression, anxiety, and suicidal ideation [32]. Poor sleep quality is correlated with decreased gray matter in the prefrontal cortex, a region responsible for emotional regulation [33]. Studies show that insomnia symptoms exacerbate depressive symptoms over time and contribute to poor treatment response in mood disorders [34]. Neurocognitive decline, particularly in attention, memory, and executive function, has also been observed in individuals experiencing chronic sleep deprivation [35]. Adolescents and young adults are particularly vulnerable to mood and behavior disorders when suffering from sleep fragmentation or insufficient sleep [36].

Metabolic Health Outcomes: Several observational and interventional studies point to a robust association between inadequate sleep and metabolic dysregulation. Individuals with poor sleep experience increased insulin resistance, elevated fasting glucose, and altered leptin and ghrelin levels, which contribute to appetite dysregulation and obesity [37]. Sleep restriction has been linked to metabolic syndrome components, including dyslipidemia, abdominal obesity, and impaired glucose tolerance [38]. Longitudinal studies show that habitual short sleepers are at higher risk for developing Type 2 diabetes and metabolic syndrome [39].

Cardiovascular Health Outcomes: Epidemiological evidence shows that chronic sleep disturbances contribute significantly to elevated blood pressure, impaired endothelial function, and increased heart rate variability [40]. Sleep apnea and insomnia are both risk factors for hypertension and ischemic heart disease. Studies indicate that sympathetic nervous system overactivation in insomniacs leads to persistent elevation in nocturnal blood pressure, predisposing individuals to left ventricular hypertrophy and other cardiac dysfunctions [41].

Immune System and Inflammation: Chronic sleep restriction is associated with a proinflammatory state as evidenced by increased C-reactive protein (CRP), IL-6, and TNF-alpha levels in sleep-restricted subjects [42]. They are also predictive of cardiovascular and autoimmune disease risk. Vaccination analysis indicates that worse sleepers have lower antibody titers post-immunization, linked to deficiencies in adaptive immune function [43]. Perturbations of the sleep-wake cycle have also been demonstrated to modulate immune surveillance, and thus, may modulate predisposition to infection, as well as chronic diseases [44].

Population-Level Variations and Vulnerable Populations:

Older adults, night and shift workers, and those with chronic stress exhibit the worst sleep quality, and are also most predisposed to systemic metabolic and inflammatory dysregulations [45]. Importantly, gender-specific pattern differences have been described, with women suffering from severe sleep disturbances having higher rates of depression and autoimmunity, while men exhibit increased risk for cardiovascular disease [46].

Interventional outcomes: Various behavioral interventions, for example, Cognitive Behavioral Therapy for Insomnia (CBT-I), have shown effectiveness in enhancing and preventing worsening of sleep and reducing the psychological and physical related symptomatology [47]. Pharmacological interventions such as melatonin supplementation and selective hypnotics have demonstrated short-term benefits yet are frequently constrained by side effects and risks of dependence. Mindfulness, exercise and digital cognitive interventions are beneficial for sleep problems and co-morbid health problems [48].

These findings collectively highlight the pervasive consequences of poor sleep quality and underline the necessity of integrating sleep assessments in routine clinical evaluations across disciplines.

The cumulative evidence from this systematic review indicates that poor sleep quality profoundly affects multiple domains of health, with cascading psychological and physiological consequences. This multifactorial impact underscores sleep as not merely a restorative state but a foundational biological function regulating mental equilibrium, metabolic homeostasis, cardiovascular integrity, and immunologic competence [49].

One of the most pressing concerns revealed by the literature is the bidirectional relationship between poor sleep and mental illness. While insomnia may precede and potentiate the development of depression and anxiety, these conditions in turn perpetuate fragmented or insufficient sleep, creating a self-reinforcing pathological loop. Neural imaging studies have demonstrated hypoactivation in the dorsolateral prefrontal cortex and limbic hyperactivation in insomniacs, suggesting impaired top-down emotional regulation [50]. Moreover, sleep deprivation exacerbates amygdala reactivity and impairs synaptic plasticity, mechanisms strongly implicated in mood instability and cognitive impairment [51].

On a physiological level, chronic poor sleep has emerged as a significant contributor to metabolic syndrome and cardiovascular dysfunction. Through the disruption of endocrine regulators such as leptin and ghrelin, inadequate sleep contributes to overeating, weight gain, and impaired insulin signaling. This dysregulation is potentiated by nocturnal sympathetic overactivity, leading to elevated blood pressure, arterial stiffness, and reduced heart rate variability [52]. The integration of these factors substantially elevates the risk of myocardial infarction, stroke, and cardiovascular mortality.

Another key pathway mediating the effects of sleep on chronic disease is inflammatory cascades that are initiated during sleep deprivation. Pro-inflammatory cytokines, including IL-6 and TNF-α, are associated not only with systemic inflammation, but also with mood disorders, insulin resistance, and atherosclerosis [53]. Furthermore, aberrant immune responses can contribute to a failure of host defense, making the host more susceptible to infections, preventing full vaccine efficacy, and hampering tissue repair. It is obviously the case that, therefore sleep, is essential not only for innate also for adaptive immunity.

Notably, the influence of socio-economic status in form of work related stress, environmental noise, screen activities and non-fixed wake times is particularly detrimental to sleep timing in low income and urban populations. Related are the data showing that shift workers and health care workers (both groups of workers having a high prevalence of circadian misalignment) have rates of hypertension, obesity, and burnout out of proportion to their normal counterparts [54]. There are also gender and age moderators of sleep-related outcomes; specifically, post-menopausal women are at a greater risk for cardiovascular disease associated with insomnia, whereas adolescents experiencing school-related stress are at a risk for long-term neuropsychological effects [55].

Public health campaigns that focus on sleep hygiene lack implementation, despite compelling evidence for its cost effectiveness and effect. Although CBT-I still represents the gold standard for non-pharmacological treatment, access barriers (eg, provider shortage, lack of public awareness) persist. In addition, wearable devices and digital health platform are promising yet under-assessed tools for sleep monitoring and behavior management [56].

In light of this evidence, multidisciplinary collaboration among sleep researchers, mental health professionals, endocrinologists, and public health policymakers is essential. Future studies must prioritize longitudinal analyses with robust sleep measurements and objective biomarkers to strengthen causal inferences. Additionally, clinical guidelines must be updated to integrate routine sleep assessments in primary care settings to enable early identification and intervention [57-58].

In conclusion, this systematic review underscores the pervasive and multidimensional consequences of poor sleep quality on human health, spanning psychological, metabolic, cardiovascular, and immune domains. The robust associations between sleep disturbances and increased risk of depression, anxiety, obesity, hypertension, Type 2 diabetes, and compromised immune response point to sleep as a fundamental pillar of overall health. Mechanistically, dysregulation of the HPA axis, inflammation, circadian disruption, and neuroendocrine imbalance emerge as central pathways linking inadequate sleep to disease development. Vulnerable populations such as shift workers, the elderly, and individuals with chronic stress require targeted interventions. Despite the availability of effective non-pharmacologic treatments like CBT-I and mindfulness-based strategies, access remains limited and awareness underdeveloped. As the burden of sleep-related disorders continues to rise globally, it is imperative that sleep health becomes an integrated component of routine healthcare practice, public health campaigns, and preventive medicine frameworks. Prioritizing sleep hygiene, early diagnosis, and multidisciplinary treatment approaches will be key to improving long-term health outcomes and quality of life across populations.

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