1 Pain in Motion Research Group (PAIN), Department of Physiotherapy, Human Physiology and Anatomy, Faculty of Physical Education & Physiotherapy, Vrije Universiteit Brussel, Brussels, Belgium.
2 Musculoskeletal Rehabilitation Research Group, Department of Rehabilitation Sciences, Faculty of Movement and Rehabilitation Sciences, KU Leuven, Leuven, Belgium.
3 Brain, Body and Cognition (BBCO), Faculty of Psychology and Educational Sciences, Vrije Universiteit Brussel (VUB), Brussels, Belgium.
4 Pain in Motion International Research Consortium.
5 VItal Signs and PERformance Monitoring (VIPER), LIFE Department, Royal Military Academy, Brussels, Belgium.
6 Department of Psychology, Faculty of Arts and Science, Concordia University, Montreal, Quebec, Canada.
7 Sleep, Cognition and Neuroimaging Laboratory, Department of Health, Kinesiology and Applied Physiology, Concordia University, Montreal, Quebec, Canada.
8 Centre de Recherche de, l'Institut Universitaire de Gériatrie de Montréal (CRIUGM), Montreal, Quebec, Canada.
9 The Leuven Centre for Algology and Pain Management, University Hospitals Leuven, Leuven, Belgium.
10 Department of Physical Medicine and Physiotherapy, University Hospital Leuven, Leuven, Belgium.
11 Department of Psychology, University of Warwick, Coventry, UK.
12 Division of Rheumatology, University Hospitals Leuven, Leuven, Belgium.
13 Pijnpraxis.be, Practice for Pain Management, Leopoldsburg, Belgium.
14 Department of Health and Rehabilitation, Unit of Physiotherapy, Institute of Neuroscience and Physiology, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden.
15 Chronic Pain Rehabilitation, Department of Physical Medicine and Physiotherapy, University Hospital Brussels, Brussels, Belgium.
16 Research Foundation Flanders (FWO), Brussels, Belgium.
17 Department of Rehabilitation Sciences and Physiotherapy, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium.
18 Department of Neurosurgery and Radiology, University Hospital Brussels, Brussels, Belgium.
19 Stimulus Research Group, Vrije Universiteit Brussels, Brussels, Belgium.
20 Center of Neurosciences, Vrije Universiteit Brussels, Brussels, Belgium.
21 Brussels University Consultation Center (BRUCC), Faculty of Psychology and Educational Sciences, Vrije Universiteit Brussel (VUB), Brussels, Belgium.
22 Laboratoire de Psychologie Médicale et d'Addictologie (ULB312), Department of Psychiatry, Brugmann University Hospital, Université Libre de Bruxelles (ULB) and Vrije Universiteit Brussel (VUB), Brussels, Belgium.
23 Nuffield Department for Surgical Sciences, University of Oxford, Oxford, UK.

Chronic spinal pain is associated with fragmented sleep, yet the neurophysiological mechanisms linking the two remain unclear. Given their role in sensory gating and sleep stability, sleep spindles may represent a key mechanism connecting chronic spinal pain and sleep fragmentation. This study examined whether pain intensity over the past 4 weeks was associated with sleep spindle density and related characteristics in individuals with chronic low back or neck pain and chronic insomnia disorder. Data from 120 participants, classified by self-reported pain severity (mild, moderate, severe), were compared on spindle density derived from C4-A1A2 and F4-A1A2 EEG channels. No significant group differences were found in spindle density (respectively, C4: p = 0.372 and F4: p = 0.744). Regression analyses adjusted for age, sex and medication use showed that pain intensity did not significantly predict spindle density (p > 0.05). However, exploratory regression models revealed small but statistically significant associations between pain intensity and both spindle power (p = 0.047) and amplitude (p = 0.038) on the C4 channel. These findings suggest that while spindle density does not vary meaningfully with pain intensity in this clinical population, alterations in spindle power and amplitude may reflect pain-related modulation of thalamocortical activity. This highlights the complexity of pain-sleep interactions in the presence of chronic spinal pain and insomnia. More nuanced, temporally resolved assessments of pain are needed to clarify the dynamic interactions between chronic pain, insomnia, and sleep microarchitecture.