## 1 Introduction  
Fibromyalgiagia is a chronic musculoskeletal pain syndrome characterized by central sensitization, a process through which the central nervous system becomes hyperreactive and amplifies nociceptive signals, leading to widespread pain and heightened pain perception [1; 2]. Beyond persistent pain, Fibromyalgiagia syndrome manifests as a complex, multifaceted condition involving fatigue [3; 4], morning stiffness [5], sleep disturbances [6], and cognitive impairments, such difficulties in regard to concentration and memory deficits [7]. Although it predominantly affects women aged 20-50 years [8], it is also observed in men [9], adolescents [10], and older adults [11]. The etiology of Fibromyalgiagia is multifactorial, encompassing genetic predisposition [12], chronic stress [13], mood disorders [14], and associations with rheumatic diseases (e.g., rheumatoid arthritis [15], lupus [16]) and infectious agents (e.g., Epstein-Barr virus [17]). Globally, Fibromyalgiagia affects approximately 2.1% of the population, with a slightly higher prevalence in Europe (2.31%) and Spain (2.4%) [18; 19]. The syndrome significantly diminishes the patient's quality of life (QoL), affecting physical, emotional, and social domains, while also imposing a substantial socioeconomic burden, due to increased healthcare usage and work absenteeism [20; 21]. The standard approach to Fibromyalgiagia management is multidisciplinary, integrating pharmacological treatments, including analgesics, antidepressants, and anticonvulsants, with structured physical exercise, cognitive-behavioral therapy, and patient education [22; 23; 24]. While these interventions can alleviate symptoms and enhance daily functioning, their efficacy varies among individuals, and adherence to treatment regimens is often limited. Additionally, the strong placebo effect observed in regard to Fibromyalgia therapies underscores the need for novel, evidence-based interventions that offer reliable symptom relief. Photobiomodulation therapy (PBMT) has recently emerged as a promising complementary treatment for Fibromyalgiagia and other chronic pain conditions [25; 26]. This non-invasive technique uses low-level lasers or light-emitting diodes (LEDs) to deliver light within specific wavelengths, typically within the red to near-infrared range (600-1100 nm), to target tissues. These wavelengths are selected based on their proven ability to penetrate tissue effectively and interact with mitochondrial chromophores, particularly cytochrome c oxidase (CCO), thereby modulating cellular processes that may reduce pain and inflammation, while promoting tissue repair [27]. Mechanistically, PBMT has been shown to enhance mitochondrial ATP production, reduce oxidative stress, and modulate gene expression related to pain signaling pathways, contributing to its potential therapeutic effects [28]. Preliminary findings suggest that PBMT may alleviate Fibromyalgiagia-related symptoms, such as chronic pain, fatigue, and psychological disturbances, including kinesophobia and reduced self-efficacy. However, given the complex and multifactorial nature of fibromyalgia, characterized by heterogeneous symptom profiles and diverse pathophysiological mechanisms, the current evidence supporting PBMT remains inconclusive and poorly understood. This complexity contributes to the wide variability in PBMT protocols, with no clear consensus regarding optimal wavelengths, session durations, treatment frequencies, or long-term safety [29; 30]. In this context, a systematic review is warranted to critically appraise and synthesize the available evidence, identify methodological gaps, and evaluate the consistency and quality of outcomes reported across studies. Such a review is essential to guide future research, inform clinical practice, and assess the potential for developing standardized, evidence-based treatment protocols.