Pain and sleep are closely linked physiological processes that support overall health and functioning. Increasing evidence shows a dynamic, bidirectional relationship: poor sleep increases pain sensitivity, while chronic pain disrupts normal sleep. Both conditions are highly prevalent and significantly impair quality of life, making them major public health concerns.

Chronic pain, defined as pain lasting more than three months, affects around 17% of adults in Spain. Insomnia, though common, is harder to define due to its overlap with medical and psychiatric conditions. It may present as a symptom, a syndrome, or a formal sleep disorder, leading to variability in prevalence estimates.

Current diagnostic criteria, including DSM-5 and the International Classification of Sleep Disorders, Third Edition (ICSD-3), adopt the unified concept of chronic insomnia disorder, defined as persistent difficulty with sleep initiation, maintenance, or quality, despite adequate opportunity for sleep, and associated with daytime impairment.

ICSD-3 distinguishes three types of insomnia:

• Chronic insomnia disorder: symptoms ≥3 times/week for ≥3 months;
• Short-term insomnia disorder: symptoms lasting less than 3 months;
• Other insomnia disorder: symptoms not meeting criteria for the above.

About one-third of the general population reports insomnia symptoms. However, when both night symptoms and daytime impact are considered, the prevalence of chronic insomnia disorder is estimated at 6-10%. Women are more frequently affected, with a female-to-male ratio of about 1.4:1.

Despite this, few studies have assessed insomnia specifically in people with chronic non-cancer pain. This gap is important, as untreated insomnia may worsen pain and reduce treatment efficacy, reinforcing a vicious cycle.

This observational study (PainSomnia) aims to estimate the prevalence of chronic insomnia among adults with chronic non-cancer pain. The results will help support integrated, individualized treatment approaches that address both sleep and pain in this high-risk population.

Sleep and pain are two biologically and evolutionarily significant processes that play a fundamental role in maintaining health and protecting bodily function. These processes interact through a delicate physiological balance. Although the exact mechanisms underlying the relationship between pain and sleep are still under investigation, current evidence supports a dynamic and bidirectional interaction between the two. Pain can disrupt sleep and reduce its capacity to provide necessary physiological and mental recovery. Conversely, inadequate or fragmented sleep can worsen existing pain conditions or contribute to the development of new pain syndromes, further impairing sleep continuity and quality.

Due to their high prevalence and impact on health and daily functioning, chronic pain and insomnia represent two major public health challenges and are associated with substantial economic and social costs.

Chronic Pain Chronic pain is typically defined as pain that persists for longer than three months and is associated with significant emotional distress, functional impairment, or both. The Pain Proposal, a European initiative aimed at understanding the burden of chronic pain, estimated that approximately 19% of the European population is affected by chronic pain. In Spain, the estimated prevalence is slightly lower, at around 17%. The economic burden of chronic pain is considerable-estimated at over €300 billion annually across Europe, accounting for roughly 1.5% to 3% of GDP. In Spain, the national cost is estimated at €16 billion annually, or approximately 2.5% of GDP.

Chronic pain has a profound negative impact on patients' quality of life, affecting physical health, emotional well-being, and social, familial, and occupational functioning. Comorbidities are common. In Spain, approximately 47% of patients with chronic pain also experience depression, and 50% report sleep disturbances. Sleep disturbances in this context encompass a range of conditions, including difficulties with sleep initiation and maintenance, as well as abnormal events during sleep such as sleep apnea, restless legs syndrome, and insomnia.

Insomnia The diagnostic criteria for insomnia have undergone significant revisions in recent years. Both the DSM-5 and the ICSD-3 have simplified and unified the diagnostic categories, moving toward a more clinically relevant concept of chronic insomnia disorder.

According to ICSD-3, chronic insomnia is characterized by persistent difficulties in initiating or maintaining sleep, or experiencing restorative sleep, occurring at least three times per week for at least three months, despite adequate sleep opportunity and conditions. These disturbances must cause clinically significant distress or impairment in social, occupational, educational, academic, or other important areas of functioning. The thresholds used to define clinical significance-such as sleep onset latency or wakefulness after sleep onset greater than 20 minutes in youth or 30 minutes in older adults-are generally based on subjective reports and clinical observation.

Chronic Pain and Insomnia: A Bidirectional Relationship Historically, insomnia was viewed as a secondary consequence of chronic pain. However, more recent research highlights a bidirectional relationship, where each condition may contribute to and exacerbate the other. Pain can fragment sleep, while disrupted sleep reduces pain thresholds and can trigger spontaneous pain episodes.

Insomnia may act as both a trigger and a perpetuating factor in chronic pain. A landmark sleep deprivation study conducted in 1999 at the University of Washington found that women subjected to selective sleep deprivation exhibited decreased pain tolerance. Since then, multiple studies have confirmed that individuals with insomnia exhibit heightened sensitivity to experimental pain stimuli.

There is strong evidence that short or disrupted sleep can induce hyperalgesia. Additionally, chronic sleep loss has been shown to predict the onset or worsening of chronic pain conditions. Longitudinal studies further support the idea that sleep problems in individuals without existing pain may significantly increase the risk of developing chronic pain syndromes.

The central nervous system has built-in mechanisms to preserve sleep, including neural filtering at the brainstem and descending inhibitory pathways designed to suppress the arousing effects of incoming sensory information. However, pain can override these mechanisms, disrupting both the continuity and architecture of sleep. The extent of this disruption depends on both the intensity of nociceptive stimuli and the individual's susceptibility to sleep fragmentation.

Clinical Relevance and Research Gap Patients with chronic pain are at significantly higher risk for insomnia compared to individuals without pain. The interaction between chronic pain and chronic insomnia is best understood as a vicious cycle in which each condition amplifies the other.

Current clinical guidelines advocate for a multidisciplinary approach to managing chronic pain, which includes addressing associated conditions such as insomnia. Failing to treat insomnia in this population can create a major barrier to effective pain management.

Despite the high co-occurrence of these conditions, no studies to date have quantified the prevalence of chronic insomnia among patients with chronic non-cancer pain in the Spanish population. Although prior studies have estimated the prevalence of sleep disturbances in chronic pain populations to range from 50% to 90%, these estimates vary widely depending on definitions used and have not been validated in local cohorts.

Understanding the true burden of insomnia in this population is essential for developing targeted therapeutic strategies that can alleviate symptoms and prevent deterioration in quality of life. This study, PainSomnia, is designed to fill this knowledge gap by providing reliable, population-specific data on the prevalence of chronic insomnia in adults with chronic non-cancer pain.

The results of this study are expected to inform the design of integrated care pathways and public health policies, ensuring that comorbid insomnia is identified early and managed appropriately in individuals with chronic pain.

Inclusion Criteria:

• Aged 18 years or older
• Either biological sex
• Diagnosed with chronic non-cancer pain
• Receiving care in a pain unit or by a pain specialist
• Able to understand and participate in the study
• Who sign informed consent

Exclusion Criteria:

• Deemed unable or unlikely to cooperate adequately with study procedures at the discretion of the pain specialist or investigator

Adult non-cancer chronic pain patients receiving care in outpatient settings from a pain specialist or within a dedicated pain unit.

IPD will be made available by the principal investigator upon reasonable request and subject to approval, in accordance with applicable data protection and ethical guidelines.

IPD will be made available 6 months after study completion and will remain available for up to 5 years upon request.

Qualified researchers may request access to de-identified IPD for secondary analyses that are consistent with the original study objectives. Requests must include a research proposal and data use agreement, and will be reviewed by the principal investigator. Data will be shared in compliance with applicable ethical and legal standards.