Melatonin is controlled by a light-sensitive pathway that runs from your eyes to a tiny gland deep in your brain. During darkness, this pathway switches on and triggers melatonin production; during light exposure, it switches off. The system is remarkably precise: nighttime blood levels typically reach 80 to 120 pg/mL, while daytime levels drop to just 10 to 20 pg/mL. Nearly 80% of all melatonin is synthesized at night.
The Signal Starts in Your Eyes
Your retinas contain specialized light-detecting cells called intrinsically photosensitive retinal ganglion cells, or ipRGCs. These cells don’t help you see images. Instead, they measure ambient brightness and relay that information to your brain’s master clock, a small cluster of neurons in the hypothalamus called the suprachiasmatic nucleus (SCN). The ipRGCs are most sensitive to blue light around 480 nm, which is the wavelength most abundant in daylight and common in screens.
What’s interesting is that the eye’s response to light changes over time. During the first 90 minutes or so of light exposure, regular cone cells in the retina (the ones tuned to short and medium-to-long wavelengths) contribute about equally to melatonin suppression. But as light exposure continues beyond that initial window, the ipRGCs gradually take over. By the second half of a sustained light exposure, melanopsin-driven ipRGCs account for 79 to 92% of the suppression signal. This means brief flashes of light and prolonged exposure actually activate different parts of your visual system.
From Brain Clock to Pineal Gland
The SCN doesn’t produce melatonin itself. Instead, it sends signals through a chain of nerve connections that ultimately reach the pineal gland, a pea-sized structure near the center of the brain. During daylight, the SCN actively suppresses this pathway. When darkness falls and the light signal from your retinas drops, the SCN releases its brake.
The final step in this relay involves nerve fibers that release norepinephrine directly onto the cells of the pineal gland (called pinealocytes). Norepinephrine activates receptors on these cells, which flips the switch for melatonin production. This is why certain blood pressure medications called beta-blockers, which block these same receptors, can cut melatonin output roughly in half. One study of patients on long-term beta-blockers found their melatonin metabolite levels dropped from a median of 24.0 to 12.8 micrograms per 24 hours compared to patients not taking the drugs.
How Your Body Builds Melatonin
Melatonin is built from serotonin, which itself comes from the amino acid tryptophan (found in protein-rich foods). Once the pineal gland gets the “go” signal at night, an enzyme called AANAT converts serotonin into a precursor molecule. A second enzyme then converts that precursor into melatonin. The speed of the entire process hinges on AANAT: its activity surges at night and drops during the day, making it the true bottleneck in melatonin production. When light hits your eyes at the wrong time, the chain of events that activates AANAT is interrupted, and melatonin synthesis stalls.
Why Blue Light at Night Matters
Light in the 460 to 500 nm range (blue to cyan) is the most potent suppressor of melatonin. This is the range where ipRGCs are most sensitive, peaking right around 480 nm. Evening exposure to bright light in this range lowers melatonin levels and delays your internal clock. One controlled study found that LED lighting engineered to boost output at 480 nm reduced morning melatonin levels by about 22% compared to conventional lighting, even at moderate indoor illuminance levels of 150 to 500 lux at eye level.
This is why nighttime screen use and bright indoor lighting can meaningfully shift your sleep timing. It’s not light in general that’s the problem. It’s the spectral composition, specifically how much energy falls in that blue-cyan window.
How Melatonin Changes With Age
Melatonin production declines across the lifespan, and pineal gland calcification is a major reason why. Calcium deposits gradually accumulate in the pineal gland over decades. Visible calcification is present in only about 2% of children under 10, but rises to 32% in teenagers, 53% in people in their twenties, and 83% in those over 30. In some animal species, calcification eventually reaches 100%.
The calcified portions of the gland no longer produce melatonin effectively. Blood and saliva melatonin levels correlate positively with the uncalcified portion of the gland and negatively with the size of the calcified area. This gradual loss of melatonin output contributes to the sleep difficulties many people experience as they get older, including lighter sleep, more frequent awakenings, and earlier wake times. Beyond sleep, reduced melatonin also means less of its antioxidant protection in the brain, which may play a role in neurodegenerative conditions.
There’s also a developmental component at the other end of life. A second decline in production capacity happens because it was never fully established: newborns don’t produce melatonin on a reliable circadian rhythm until around 9 weeks of age. Before that point, their sleep-wake patterns aren’t governed by the same light-dark melatonin cycle that adults rely on.
Substances That Disrupt Melatonin Control
Because the final trigger for melatonin release depends on norepinephrine binding to specific receptors on the pineal gland, anything that blocks those receptors will reduce output. Beta-blockers are the most well-documented example, with long-term use associated with roughly a 50% decrease in melatonin synthesis. This effect is more clinically significant in people who already have low baseline melatonin levels, where the additional suppression can tip the balance toward insomnia.
Caffeine also influences the system, though through a different mechanism. It interferes with the buildup of sleep pressure rather than acting directly on the pineal pathway. The practical result, however, is similar: consuming caffeine in the hours before bed delays the timing of your internal clock and reduces the effective window during which melatonin can do its job.
Bright indoor lighting, shift work, and jet lag all disrupt melatonin control by sending conflicting light signals to the SCN. The system evolved to respond to a clean on-off cycle of sunlight and darkness. When that cycle is blurred by artificial light, the timing and amplitude of melatonin release become less precise, which is why consistent light and dark patterns remain the single most effective tool for keeping melatonin regulation on track.