A reference for the terms that come up across the site. Each entry links out to the pillar guide where the concept is unpacked in depth.
This page is updated as new terms enter the rotation. If we use a term without defining it and it is not here, please flag it via the contact page and we will add it.
Adenosine
A neurochemical that accumulates in the brain during waking hours and signals sleep pressure. Caffeine blocks adenosine receptors, which is why caffeine wakes you up — and why caffeine after 2 p.m. wrecks your sleep latency. See Better Sleep Without Drugs.
Adaptogen
A category of botanical compounds (ashwagandha, rhodiola, eleuthero, etc.) believed to modulate the body's stress response. Evidence is mixed; effect sizes are usually small. The category is heavily marketed in longevity supplement stacks.
BAT (Brown Adipose Tissue)
Metabolically active fat tissue that generates heat in response to cold exposure. The proposed mechanism behind cold plunging's effect on metabolic health. See the Cold Plunge Guide.
Biological age
A composite estimate of how "old" your body is based on biomarkers (epigenetic methylation, blood markers, etc.), as distinct from chronological age. The aspirational target of most longevity protocols.
Box breathing
A breathing protocol — inhale 4 seconds, hold 4, exhale 4, hold 4 — used to acutely shift autonomic state toward parasympathetic. Effective for transient stress reduction.
CGM (Continuous Glucose Monitor)
A wearable that measures interstitial glucose continuously, originally developed for diabetics, now used by non-diabetics for metabolic feedback. See Best CGM for Non-Diabetics.
CoA (Certificate of Analysis)
Third-party batch testing documentation for a supplement, verifying that the product contains what the label claims. Reputable supplement brands publish per-batch CoAs. See Best NMN Supplement.
Cold shock proteins
A family of proteins (notably RBM3) upregulated by cold exposure. Proposed mechanism for some of cold therapy's longer-term effects; still mostly preclinical. See the Cold Plunge Guide.
Contrast therapy
Alternating between hot (sauna) and cold (plunge) exposures within a single session. See the Sauna + Cold Plunge protocol.
CWI (Cold-Water Immersion)
Clinical term for cold plunging. The literature uses CWI; the marketing uses "cold plunge" or "ice bath." Same thing.
Dopamine
A neurotransmitter associated with motivation, reward, and movement. Cold exposure transiently raises dopamine ~250%, which is part of the "feel great after" effect of cold plunging.
DOMS (Delayed-Onset Muscle Soreness)
Soreness peaking 24-48 hours after exercise, caused by microtears in muscle tissue. Cold-water immersion modestly reduces DOMS (~15-20% in validated scales).
EMOM (Every Minute On the Minute)
A training structure where you complete a set number of reps at the start of every minute. Used in CrossFit and conditioning work.
Endothelial function
The responsiveness of the inner lining of blood vessels to dilation signals. A validated marker of cardiovascular health. Improves with chronic sauna use and zone 2 cardio.
Flow-mediated dilation (FMD)
The clinical measurement of endothelial function. Measured ultrasonically. The reference standard for evaluating vascular interventions.
Heat shock proteins
A family of proteins (HSP70, HSP90) upregulated by heat exposure. One proposed mechanism for sauna's longevity-associated effects. See Sauna for Heart Health.
Hormesis
The biological principle that a low-dose stressor can produce a beneficial adaptive response. The framing under which cold exposure, sauna, fasting, and exercise are all related.
HRV (Heart Rate Variability)
The millisecond variation in time between consecutive heartbeats. A passive readout of autonomic nervous system state. Higher = better recovery, within a single person. See HRV Explained.
Hypertrophy
Muscle growth from training-induced damage and protein synthesis. Cold-water immersion within 4 hours post-lift may blunt the hypertrophy signal. See Cold Plunge Guide and Recovery Stack for Athletes.
Infrared (near, mid, far)
Subdivisions of infrared light by wavelength. Near-IR (~700-1100nm) penetrates skin; far-IR (~3000-100000nm) heats surface tissue. Most infrared saunas use a mix. See Home Sauna Guide.
Irradiance
The power of light per unit area at a given distance, measured in mW/cm². The fundamental dose unit for red-light therapy. See Red Light Therapy Guide.
Joule
A unit of energy. Used for red-light therapy dosing as J/cm² (energy delivered per unit area). Treatment doses are typically 5-60 J/cm².
KIHD (Kuopio Ischemic Heart Disease Study)
The Finnish longitudinal cohort from which most long-term sauna-and-cardiovascular data is drawn. See Sauna for Heart Health.
NAD+ (Nicotinamide Adenine Dinucleotide)
A coenzyme essential for cellular energy metabolism and DNA repair. Declines with age. The aspirational target of NMN/NR supplementation. See Best NAD Supplement.
NMN (Nicotinamide Mononucleotide)
A direct biosynthetic precursor to NAD+. Marketed heavily as a longevity supplement; the human RCT evidence is preliminary. See Best NMN Supplement.
NR (Nicotinamide Riboside)
A precursor one step before NMN in the NAD+ biosynthesis pathway. Better-studied in humans than NMN, with a longer commercial track record. Sold as Tru Niagen and Elysium Basis.
NSF Certified for Sport
A third-party certification verifying a supplement is free of banned substances and contains what the label claims. The standard for athletes.
Polysomnography (PSG)
The clinical gold standard for sleep staging. Used in sleep labs. Wearable sleep tracking is validated against PSG, never the other way around.
Proprietary blend
A supplement-label format where multiple ingredients are listed under a single total weight, without per-ingredient doses. Industry-standard practice; also opaque and bad practice. Avoid where you can.
REM (Rapid Eye Movement)
The sleep stage associated with dreaming, memory consolidation, and emotional processing. Typically 20-25% of total sleep. Suppressed by alcohol and many medications.
RMSSD (Root Mean Square of Successive Differences)
The most common consumer HRV metric — the statistical measure underlying the number in your Oura or Whoop app. See HRV Explained.
Sleep latency
The time it takes to fall asleep after lights out. Healthy range: 10-20 minutes. Caffeine, screens, alcohol, and bright evening light all lengthen sleep latency.
Slow-wave sleep (SWS / deep sleep)
The deepest stage of non-REM sleep. Critical for physical recovery and growth-hormone release. Typically 15-25% of total sleep. Suppressed by alcohol, sometimes augmented by Eight Sleep cooling.
Sympathetic / Parasympathetic
The two arms of the autonomic nervous system. Sympathetic = "fight or flight" (high arousal). Parasympathetic = "rest and digest" (recovery). HRV reflects the balance between them.
TRE (Time-Restricted Eating)
A form of intermittent fasting where eating is confined to a window (typically 8-10 hours). The most-studied variant of fasting in humans. Mixed evidence for cardiometabolic outcomes.
Vasoconstriction / Vasodilation
The narrowing or widening of blood vessels. Cold causes vasoconstriction; heat causes vasodilation. Repeated cycles train the vasculature — the mechanism behind contrast therapy.
Wim Hof Method
A breathing-and-cold-exposure protocol popularized by Wim Hof. Useful for some, oversold by most of the secondary internet. See the Cold Plunge Guide for what the research actually supports.
Zone 2 cardio
Aerobic exercise at an intensity where you can still hold a conversation (~60-70% of max heart rate). The most-studied training zone for mitochondrial adaptation and longevity. The single best lever for raising baseline HRV.
Missing a term?
Email Trevor and we will add it. The glossary grows as the site does.
Last reviewed · May 2026 · The full canonical version of this page lives here.