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 imagin…