Examining the links between chronic low back pain and insomnia

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 will be utilized in the study, all analgesics that are only taken occasionally should be refrained from during the 24 hours preceding the main test day (pre-test instructions).

Specific inclusion criteria for "pain-free" groups:

* Currently pain free, defined as having no pain that affects daily living/functioning and/or which has led to a consultation with a healthcare practitioner.
* No episode of low back pain that interfered with daily functioning/living and/or which led to a consultation with a healthcare practitioner within 12 months from the time of inclusion.
* No history of chronic low back pain.

Specific inclusion criteria for "chronic low back pain" groups:

* Low back pain reported on most days (≥50%), every week, for more than 3 months.
* Low back pain-related emotional disress OR functional disability rated as ≥3 out of 10.

Specific inclusion criteria for "good sleeper" groups:

* Stable sleep phase within 10 PM and 10 AM (assessed using the Pittsburgh Sleep Quality Index).
* Average habitual total sleep time between 7 and 9 hours (assessed using the Pittsburgh Sleep Quality Index).
* A Pittsburgh Sleep Quality Index ≤5.
* An Insomnia Severity Index ≤7.
* An Epworth Sleepiness Scale ≤10.
* Sleep efficiency ≥85% during a one-night, laboratory-based polysomnography screening.
* No episode of sleep disturbances that interfered with daily functioning/living and/or which led to a consultation with a healthcare practitioner within the preceding 12 months from the time of inclusion.
* No history of chronic insomnia.

Specific inclusion criteria for "chronic insomnia" groups:

* Chronic insomnia according to the ICSD-3-TR criteria (assessed using an ICSD-3-TR-adapted version of the Pittsburgh Sleep Quality Index).
* An Insomnia Severity Index ≥15.