Studying the Neurobiological Mechanisms of Non-specific Chronic Low Back Pain and Chronic Insomnia: a Four-group Cross-sectional Study

General inclusion criteria (all groups): * 18-60 years old. * Ability to speak and understand the Dutch Language. * Body mass index \<30. * No smoking or nicotine use. * Low caffeine use (≤3 cups of coffee/day, ≤1 energy drink/day). * Low alcohol use (≤9 alcoholic units/week). * No use of neuro- and/or psychopharmacological treatments and/or immunosuppressive agents (including NSAIDs) with a suspected influence on neuroinflammation unless terminated within a sufficient time frame before the start of the study, or unless falling under the Pharmacological exceptions criteria (see below). * Willing to (try to) adhere to the acute pre-test restrictions, including refraining from analgesics (unless stable use), caffeine, alcohol, and/or strenuous physical activity (\>3 METs) in the 24 hours preceding the main test day. General exclusion criteria (all groups): * Shift worker. * Use of blood thinners. * (History of) Substance dependence/abuse. * Chronic pain conditions (≥3 months) other than chronic low back pain. * Severe intrinsic sleep disorders (assessed during a one-night, laboratory-based polysomnography). * Restless leg syndrome * Central or peripheral neurological disorder/condition (e.g., epilepsy, multiple sclerosis, peripheral neuropathy). * History of spinal surgery. * Major medical (e.g., cardiac disease, cancer) and/or psychiatric disorder/disease (e.g., major depressive disorder, bipolar disorder). * Claustrophobia. * Contraindications for magnetic resonance imaging (e.g., metal/cochlear implants, pacemaker). * Pregnancy or being \<12 months post-natal. Pharmacological exceptions criteria: \- Participants with chronic low back pain and/or (comorbid) insomnia will be allowed to be on a stable low dose of weak pain and/or sleep agents (e.g., Tramadol, Zolpidem) if medically prescribed specifically for their low back pain and/or insomnia, and if approved by the national medicine register of Belgium (FAMHP, Federal Agency for Medicine and Health Products). These participants will also be allowed stable and/or occasional use of low-to-moderate doses of over-the-counter medications for pain and/or sleep that are suspected to have no or minimal impact on neuroinflammation (e.g., paracetamol, melatonin). Habitual use of medications according to the above-stated criteria should be maintained across the entire study period, with the only exception being analgesics taken on an occasional basis. Specifically, because of the extensive battery of pain tests that…