Melatonin: The Sleep Hormone
Melatonin is the body's main signal that it is night. Released in darkness, it helps set the timing of the sleep–wake cycle and coordinates many daily rhythms throughout the body.
What melatonin is
Melatonin is a small hormone built from the amino acid tryptophan, which the body first converts into the neurotransmitter serotonin and then into melatonin through two further enzyme steps. The final steps are most active in darkness, which is why melatonin is sometimes called the "hormone of darkness." It is helpful to think of melatonin less as a sleeping pill and more as a clock signal: it does not force the brain into unconsciousness, but it announces that the dark portion of the day has begun and helps every tissue keep time with the outside world.
Because it is fat- and water-soluble, melatonin passes easily across cell membranes and reaches many tissues. It acts largely through dedicated melatonin receptors found in the brain's master clock, blood vessels, and other organs. This wide reach is part of why melatonin is considered a coordinating signal rather than one with a single narrow target.
Where it is produced
The melatonin that circulates in the blood and signals night comes mainly from the pineal gland, a pea-sized structure deep in the center of the brain. The pineal gland sits outside the usual blood–brain barrier, which lets the melatonin it makes enter the bloodstream quickly and spread throughout the body.
Smaller amounts of melatonin are produced locally in other tissues, including parts of the gastrointestinal tract, the retina of the eye, and certain immune cells. This locally made melatonin is generally thought to act near where it is produced rather than to set the body's overall night signal, which remains the job of the pineal gland.
What it does across body systems
- Sleep–wake timing: Rising melatonin in the evening helps open the body's "sleep gate," lowering alertness and core body temperature so that sleep can begin at the appropriate time.
- Circadian coordination: Melatonin helps synchronize daily rhythms across many organs, acting as a chemical messenger that tells peripheral clocks in the liver, gut, and elsewhere what time the central clock believes it is.
- Seasonal signaling: Because nights are longer in winter, the duration of nightly melatonin release encodes the length of the night, a signal that drives seasonal biology in many animals and is studied in humans.
- Other roles under study: Melatonin is investigated as an antioxidant and for possible roles in the immune system and metabolism. These remain areas of ongoing research rather than settled physiology.
How levels are regulated
Melatonin is regulated chiefly by light reaching the eyes, which makes it unusual among hormones. A master clock in the brain, the suprachiasmatic nucleus (SCN) of the hypothalamus, receives light information from specialized cells in the retina and relays it through a nerve pathway to the pineal gland. In darkness, the SCN allows the pineal gland to release melatonin; light — especially bright, blue-rich light — suppresses that release within minutes.
As a result, melatonin typically begins to rise a couple of hours before a person's usual bedtime, peaks in the middle of the night, and falls toward morning. The moment melatonin starts to climb under dim light is sometimes used in research as a marker of internal clock timing. Because the system answers to environmental light rather than a classic blood-borne feedback loop, melatonin is more dependent on the light environment than most hormones, and evening screen light, shift work, or travel across time zones can shift or blunt it.
Why timing matters more than amount
For many hormones, the headline question is "how much." For melatonin, the more meaningful question is often "when." A melatonin rhythm that arrives at the right clock time supports sleep at the desired hour; the same total amount released at the wrong time can leave a person sleepy by day and alert at night. This is why melatonin is usually discussed in terms of phase and timing rather than a single number.
What high or low levels can be associated with
A melatonin rhythm that is shifted or reduced can be associated with difficulty sleeping at the desired time, as seen with jet lag and shift work, where the internal clock and the social clock disagree. The natural nightly amount of melatonin also tends to be highest in childhood and to decline gradually with age, which is one reason sleep patterns often change across the lifespan.
Because melatonin is so closely tied to light and routine, a "low" reading frequently reflects light exposure at the time of sampling rather than a gland that cannot make the hormone. For this reason, melatonin patterns are interpreted in the context of a person's sleep schedule, light environment, and age rather than against a fixed normal value. These associations are qualitative; see the conditions index and the glossary for related terms.
How it is measured
Melatonin is not part of routine hormone panels. When it is studied, it is usually measured in research or specialized settings, in saliva, blood, or a urinary breakdown product, and collected at several time points so the rhythm can be seen. Samples are often gathered in the evening under controlled dim-light conditions because ordinary room light can suppress the very hormone being measured. For most people, melatonin is not a standard clinical blood test. See the blood tests overview for context.
| Timing | General pattern |
|---|---|
| Daytime | Low (illustrative; varies by individual and light exposure) |
| Evening | Rising as darkness begins (illustrative) |
| Middle of the night | Typically highest (illustrative; varies by age and environment) |
| Toward morning | Falling as the body prepares to wake (illustrative) |
The values above are deliberately qualitative. Because a single timed sample says little without knowing the light conditions and the person's usual schedule, melatonin is one of the few hormones where the shape of the curve matters far more than any one figure.
Relationships with other hormones and signals
Melatonin sits within a wider timekeeping network. It is made from serotonin, so the two share a biochemical pathway. The cortisol rhythm runs roughly opposite to melatonin's, with cortisol low at night and rising toward morning as melatonin falls, and together they help frame the body's daily energy curve. Core body temperature, alertness, and the timing of many other hormones are all coordinated by the same master clock that governs melatonin, which is why disruptions to one rhythm often ripple into the others. To explore neighboring messengers in this system, see the hormones index.
Frequently asked questions
Does melatonin make you fall asleep?
Melatonin acts more as a timing signal for night than as a sedative. It helps align the body's clock with darkness rather than forcing sleep directly.
How does light affect melatonin?
Light reaching the eyes suppresses melatonin, while darkness allows it to rise. Bright or blue-rich light at night can shift or reduce its release within minutes.
Where is melatonin made?
Mainly in the pineal gland, a small structure deep in the brain, with smaller amounts made locally in tissues such as the gut and retina.
Is melatonin a routine blood test?
No. It is not part of standard hormone panels and is usually measured only in research or specialized settings under controlled light conditions.
Why does melatonin seem to change with age?
The nightly amount tends to be highest in childhood and to decline gradually over the years, which is one reason sleep timing and depth often shift across the lifespan.
How is melatonin related to cortisol?
Their daily rhythms run roughly opposite: melatonin is high at night while cortisol is low, and cortisol rises toward morning as melatonin falls. Both are timed by the same master clock.
Sources
- MedlinePlus. Hormones. https://medlineplus.gov/hormones.html
- National Institutes of Health. https://www.nih.gov/
- Cleveland Clinic. https://my.clevelandclinic.org/