Glossary

Plain-English definitions of the technical terms used across the site. Where a term links to a protocol page, you'll find a fuller treatment there.

AASM: American Academy of Sleep Medicine — the body that publishes scoring guidelines for sleep stages used in clinical polysomnography.
Adaptogen: A non-regulatory term, coined in Soviet-era pharmacology, for compounds claimed to non-specifically increase resistance to stress. The category has no MHRA-defined meaning; evidence varies sharply by compound.
ASA: Advertising Standards Authority — the UK self-regulatory body for advertising claims. Health claims in marketing must comply with ASA rules; this site uses ASA-safe language throughout.
Autophagy: Cellular recycling of damaged organelles and proteins. Well-established biology; the specific timing and human relevance during fasting is less well-characterised than online claims suggest.
BAT (brown adipose tissue): Heat-producing fat tissue that burns calories to generate warmth. Activated by cold exposure. Small in absolute calorie terms in adult humans.
CBT-I: Cognitive Behavioural Therapy for Insomnia — the NICE-recommended first-line treatment for chronic insomnia. Not a self-help blog post; a structured 4–8 session intervention available on the NHS.
CGM: Continuous Glucose Monitor — a sensor worn on the upper arm or abdomen that reads interstitial-fluid glucose every few minutes for 10–14 days.
Circadian rhythm: The roughly 24-hour internal biological cycle governing sleep, hormone release, body temperature, and metabolism. Primarily entrained by light exposure.
Cold shock response: The involuntary gasp, hyperventilation, and tachycardia that follow sudden cold-water immersion. Habituates within 4–6 exposures. The first 1–2 minutes carry the highest cardiovascular risk.
Cycle (sleep): A roughly 90-minute repeating sequence of NREM and REM sleep stages. A normal night contains 4–6 cycles.
DKIM, SPF, DMARC: Email authentication standards. DKIM signs outgoing email; SPF declares which servers may send from a domain; DMARC tells receivers what to do with failures. Set via DNS records.
EPA, DHA: Long-chain omega-3 fatty acids found in oily fish. The two most-studied marine omega-3s.
Evidence grade: Our A–D rating of the strength of human research behind a protocol. See the full methodology.
GLP-1: Glucagon-like peptide-1 — a gut-derived incretin hormone. Released after eating; involved in insulin secretion and appetite regulation. Several prescription medicines (e.g. semaglutide) are GLP-1 receptor agonists.
HbA1c: Glycated haemoglobin — a blood marker reflecting average glucose levels over the previous ~3 months. The standard NHS measure for diabetes diagnosis and monitoring.
HRV: Heart Rate Variability — the variation in time between consecutive heartbeats. Reflects autonomic nervous system balance.
Insulin: Hormone released by the pancreas in response to rising blood glucose. Drives glucose uptake into cells, fat storage, and renal sodium retention.
Lux: A unit of illuminance — light intensity reaching a surface. Indoor lighting is typically 100–500 lux; outdoor light is 1,000–100,000 lux.
MARD: Mean Absolute Relative Difference — a standard accuracy metric for CGMs compared to laboratory plasma glucose. Modern devices achieve MARD around 7–9%.
Melanopsin: A photopigment in intrinsically photosensitive retinal ganglion cells, most sensitive to short-wavelength (~480 nm) light. The main retinal pathway for circadian entrainment to environmental light.
MHRA: Medicines and Healthcare products Regulatory Agency — the UK regulator for medicines, medical devices, and pharmacovigilance. Runs the Yellow Card scheme.
NICE: National Institute for Health and Care Excellence — the UK body that produces clinical guidelines for NHS care.
Nootropic: An umbrella term for substances claimed to enhance cognition. The category includes everything from caffeine (well-evidenced) to prescription stimulants (POMs) to grey-market compounds (legally risky). Not a regulated category in itself.
NREM: Non-rapid-eye-movement sleep — stages N1, N2, and N3 (deep / slow-wave). About 75–80% of total sleep in healthy adults.
Phase-response curve: A graph showing how the timing of a stimulus (typically light) shifts the circadian phase. Morning light advances; evening light delays.
Photobiomodulation: The clinical term for red and near-infrared light therapy. Has dermatological indications with FDA-cleared devices; broader systemic claims are weaker than marketing implies.
POM: Prescription-Only Medicine — under UK law, a medicine that may only be supplied with a valid UK prescription. Many compounds discussed in nootropic communities (modafinil, methylphenidate, piracetam) are POMs.
PSG: Polysomnography — the laboratory gold-standard sleep study, measuring EEG, EOG, EMG, respiration, and oximetry. The reference against which consumer wearables are validated.
REM: Rapid Eye Movement sleep — characterised by EEG resembling waking, paralysed skeletal muscles, and the eye movements that name the stage. Most vivid dreaming occurs in REM. Concentrated in the second half of the night.
RMSSD: Root Mean Square of Successive Differences — the most common short-term HRV metric. Reflects parasympathetic activity. The metric most consumer wearables actually report under the 'HRV' label.
SAD: Seasonal Affective Disorder — depression with a seasonal pattern, most commonly with onset in autumn or winter. Bright light therapy (10,000 lux for 30 minutes daily) is established first-line treatment.
SDNN: Standard Deviation of N-N intervals — an HRV metric capturing total variability across a recording window. Used clinically over 24-hour Holter recordings as a prognostic marker.
SHR-5: A standardised rhodiola rosea extract used in most published rhodiola RCTs. Different extracts are not equivalent.
Sleep efficiency: Percentage of time in bed spent asleep. Healthy adults typically achieve 85–95%.
Sleep latency: Time from intending to sleep to actually falling asleep. Wearables tend to underestimate latency by misclassifying quiet wakefulness as sleep.
TRE: Time-Restricted Eating — the formal name for intermittent fasting protocols where eating is confined to a defined window (e.g. 8 hours). The most-studied form is 16:8.
Vagal tone: A proxy for parasympathetic nervous system activity, often inferred from HRV metrics dominated by short-term variation (RMSSD, HF-power).
Yellow Card scheme: The MHRA pharmacovigilance system. Anyone in the UK can report suspected side effects of medicines, vaccines, supplements, e-cigarettes, or medical devices.