Melatonin

Endogenous pineal hormone, OTC supplement — but the dose and timing matter more than the molecule. For Dylan, this is a phase-shift tool, not a sleep drug. Late-chronotype migration from 2-3 AM toward midnight is a textbook indication for 0.3-0.5 mg taken 5-7 hours before target bedtime (i.e., 6-8 PM if target is midnight) — not the 3-10 mg gummy doses sold on Amazon, which target sleep onset acutely but do little for circadian phase and at chronic high doses can fragment sleep, blunt next-day cortisol, and saturate MT receptors. STRONG-CANDIDATE for his V5 phase-advance protocol, paired with morning bright light, evening blue-light blockers (Uvex SCT-Orange), and consistent wake time. NOT a recommendation to take 5 mg at bedtime.

Endogenous role. Melatonin (N-acetyl-5-methoxytryptamine) is synthesized from serotonin in pinealocytes via two enzymatic steps (AANAT, then HIOMT/ASMT). Pineal output is suppressed by retinal light input (via retinohypothalamic tract → SCN → superior cervical ganglion → pineal); in dim light it rises sharply ~2 hours before habitual sleep onset (the "dim-light melatonin onset," DLMO), peaks around 2-4 AM, and falls before waking. Melatonin is the brain's biological-night signal — it tells every clock-bearing tissue in the body what time it is on the 24-hour wheel.

Receptors.

• MT1 (MTNR1A): Gi-coupled GPCR, broadly distributed; densest in SCN, hippocampus, hypothalamus. Activation suppresses neuronal firing in the SCN's wake-promoting cells → acute sleep-promoting / wake-suppressing effect. This is the receptor that mediates the "I feel sleepy" sensation 30-60 min after a melatonin dose.
• MT2 (MTNR1B): Gi-coupled GPCR, also broadly distributed; densest in retina, SCN. Activation at the right circadian time shifts the phase of the SCN master clock — the receptor responsible for jet-lag correction, DSWPD treatment, and chronotype migration. MT2 is the circadian receptor.
• MT3 (quinone reductase 2): Not a true GPCR; an enzymatic binding site involved in detoxification and antioxidant function. Binds melatonin at much higher concentrations than MT1/MT2 — relevant for the antioxidant claims at high doses, mostly irrelevant for sleep/circadian effects.
• Nuclear ROR/RZR receptors: Bind melatonin weakly; modulate immune and circadian gene transcription. Probably matters for chronic high-dose pharmacological effects.

Phase Response Curve (PRC) — the key concept Dylan needs to understand. The same dose of melatonin produces opposite effects depending on when in the circadian cycle it's taken, because the MT2-mediated phase-shifting effect follows a sinusoidal Phase Response Curve:

• Taken in the late afternoon / early evening (~5-7 hr before habitual sleep onset, near "circadian time 9-11"): Phase-advance — the body's clock moves earlier the next day. This is the regime for treating DSWPD / late chronotype.
• Taken near habitual bedtime: Sleep-promoting via MT1 but minimal phase-shift (PRC is at zero-crossing here). This is what most people do, and it doesn't fix chronotype.
• Taken in the early morning / overnight (after the natural melatonin peak): Phase-delay — the body's clock moves later. This is why poorly-timed melatonin can make chronotype problems WORSE.
• Taken in the morning (after waking): Drowsiness without circadian benefit, plus disruption of normal cortisol awakening.

Why low dose for phase advance. The MIT group (Wurtman, Zhdanova, Lynch) demonstrated through dose-response work in the 1990s-2000s that endogenous nighttime peak plasma melatonin is roughly 60-70 pg/mL. A 0.3 mg oral dose produces plasma levels in the physiological range (peak ~100-150 pg/mL); 1 mg produces high-physiological (~300-500 pg/mL); 3-10 mg produces supraphysiological levels 10-100× the natural peak (1,000-10,000+ pg/mL). For phase-shift signaling at MT2, the dose-response curve plateaus at physiological levels — receptor saturation kicks in fast — so 0.3-0.5 mg gives the same phase-shift signal as 5 mg, with cleaner kinetics and faster clearance (half-life 30-50 min for IR melatonin). The supra-doses don't shift phase any harder; they just spill exogenous hormone outside the target receptor system, prolong elevated plasma levels into morning hours, and risk receptor desensitization on chronic use. Brzezinski 2005 meta-analysis (n=17 studies) found 0.3 mg as effective as higher doses for sleep onset latency reduction, with cleaner morning waking. Zhdanova's MIT work (2001 JCEM) showed older adults given 0.3 mg restored physiological melatonin levels and improved sleep efficiency — 3 mg overshot and produced next-morning sleepiness.

What melatonin does NOT do at any dose:

• Does not produce GABAergic sedation — no benzo-like knockout effect
• Does not cause dependence or withdrawal (no rebound insomnia on discontinuation per most studies)
• Does not significantly alter sleep architecture at physiological doses (no major REM/N3 shifts; supra-doses can cause vivid dreams via REM modulation)
• Does not directly extend total sleep time much in healthy non-circadian-disordered adults — meta-analytic effect on TST is ~8-13 minutes, on sleep onset latency ~7 minutes (Buscemi 2005, Ferracioli-Oda 2013) — small effects
• Does not work as a true hypnotic for severe insomnia — DORAs (daridorexant), Z-drugs, or trazodone outperform it for primary insomnia