Original of this article was published on ZRT Laboratory Blog. Last reviewed: May 2026.
Melatonin is a ubiquitous little hormone that comes in an indolamine package similar to serotonin, its precursor. Thanks to its unique structure, it can travel easily through the bloodstream and readily crosses the blood-brain and placental barriers. It is synthesised and used all over the body to perform a number of indispensable roles. It’s the main reason our bodies know what time it is and what they’re supposed to be doing at any given moment. It signals the transcription of genes that regulate the cell cycle, neutralise free radicals, and decrease inflammation. Its claim to fame, however, is its role in promoting sleep.
Melatonin’s Daily Curve
If we were to look at a curve representing production of melatonin over a 24-hour period, we would see a steep rise to peak levels within the first hour of sleep, with a plateau lasting until about 3–4am, followed by a drastic reduction to its daytime baseline level by around 7am.
Darkness stimulates the production of melatonin and sustains its peak level via signals that relay from the retina in the back of the eye to the suprachiasmatic nucleus of the hypothalamus in the brain, sending norepinephrine to the pineal gland to ramp up melatonin manufacture. When melatonin is released, it floods the cerebrospinal fluid, the bloodstream and surrounding tissues and acts as a potent sedative to the nervous system. It can readily enter all cells in all tissues — for some purposes, it enters via membrane receptors and sets off a cascade of phosphorylation events; for other purposes, it diffuses through the cell membrane and heads straight to the mitochondria for antioxidant duty or to the nucleus for promoting DNA transcription of specific proteins and enzymes. With high levels of melatonin on board, the body rests and repairs.
At sunrise, or the retina’s detection of light stimulation, melatonin production ramps down quickly to daytime levels. This is why it is imperative to sleep in the dark and to avoid turning on lights in the middle of the night.
How Melatonin Is Measured: Urinary MT6s
In the urine, we measure melatonin’s metabolite MT6s first thing in the morning to assess the overnight production of melatonin. MT6s shows up in the urine about 3 hours after the appearance of melatonin in the bloodstream and thus persists for about 3 hours after melatonin’s morning decline — making it a simple and accurate marker of melatonin production.
By looking at the first morning melatonin level (5am), we can see whether overnight melatonin levels were adequate. If there are reported symptoms of delayed sleep onset or other sleep disturbances as well, we could make the assessment that addressing melatonin levels might be an important part of the treatment plan.
Our Advanced Neurotransmitter & Hormone Test II measures urinary MT6s (melatonin metabolite) alongside a full panel of neurotransmitters including serotonin, dopamine, GABA and norepinephrine — providing a comprehensive picture of the neurochemical environment affecting sleep, mood and energy from a simple at-home dried urine collection.
For those who want to assess melatonin and neurotransmitters alongside a full saliva sex hormone panel, our Elite Neurotransmitters & Saliva Hormone Test combines both in a single at-home test — with specialist interpretation included to help contextualise sleep disturbances within the broader hormonal picture.
Because elevated cortisol is one of the most common suppressors of melatonin production and a key driver of disrupted sleep architecture, our All Day Cortisol Test (LCMS) measures cortisol across four time points throughout the day — helping to identify whether a dysregulated cortisol rhythm may be contributing to poor sleep onset or early morning waking.
Original of this article was published on ZRT Laboratory Blog.