The Rhythm of the Night: Melatonin
Melatonin is the hormone of darkness. Produced by the brain’s pineal gland in response to fading light, its rise signals that the body has entered “night mode.” This sets off a cascade of restorative processes that are best carried out during rest. By morning, light entering the eyes signals the brain’s master clock, the suprachiasmatic nucleus (SCN), to suppress melatonin production.
“It’s remarkable that there are only five cells between the environmental input, which is light, and the hormonal output of melatonin. That kind of direct cause and effect relationship is exceptionally rare in our complex human physiology.”
Here’s how it works: light activates a specialized group of non-visual retinal cells called intrinsically photosensitive retinal ganglion cells (ipRGCs), which communicate time-of-day information to the SCN, keeping our circadian rhythm aligned with the external world.
Melatonin serves as one of the most reliable markers of the central circadian clock, unless artificial light at night suppresses its production.
Melatonin ≠ Sleep
Let’s clarify a common misconception: melatonin is not a sleep hormone.
Its direct hypnotic effects are minimal. In fact, in nocturnal mammals, melatonin is secreted during their active period. Because it’s such a common misconception, let’s take a moment to look at what’s really happening in the brain during sleep.
Sleep is governed by two oscillating processes:
Homeostatic Mechanism: how sleepy you are is driven by how long you've been awake.
Circadian Mechanism: Often described as a clock, it’s genetically set but can be adjusted by various factors – most powerfully by light.
Both of these systems converge neurologically in the anterior hypothalamus, specifically in the ventrolateral preoptic nucleus. This is the brain’s “sleep switch” response for maintaining and initiating sleep.
While melatonin is our best biomarker of the circadian system that influences sleep, it’s not the clock itself. However, we do have a powerful, direct way to influence this system: light. And that makes circadian rhythms one of the most modifiable levers we have to improve sleep, as well as a host of other physiological processes.
Lighting the Way: Practical Tips to Regain Rhythm
Because light has such a direct and powerful influence on your central circadian clock, small changes to your daily light exposure can yield significant impact. The goal is to help your body clearly distinguish day from night, supporting your natural circadian rhythm and improving almost every physiological system.
Here are some practical, science-backed ways to start:
Get Bright Light in the Morning: Start your day with sunlight, ideally within the first hour of waking. Even on a cloudy day, stepping outside provides more light than indoor bulbs, signaling your brain that it's daytime. This shuts off lingering melatonin and helps set your internal clock earlier, making it easier to feel sleepy when night comes. Aim for 15–30 minutes of morning light exposure, without sunglasses if possible.
Keep Evenings Dim and Calm: In the hour or two before bed, try dimming lights, switching to warmer tones, and minimizing screen time. Evening light can trick your body into thinking it’s daytime, disrupting melatonin production.
Anchor Your Day with Routine: Your body takes cues from more than just light. When we eat, move, and rest all send signals that reinforce our internal clock. Try to keep regular mealtimes and include some light physical activity during the day, even just a walk or sitting near a window. Bonus tip: Sticking to a consistent wake time and time you first receive light into your eyes, even if your bedtime varies, is the best thing you can do for your circadian rhythm.
Try a “Digital Sunset”: Set a reminder an hour or two before bed to step away from bright screens and stimulating tasks. Use that time to wind down: read a book, stretch, or take a warm shower. By the time you get in bed, melatonin should be near its peak, and your body should be primed for sleep. Creating a gentle pre-sleep routine helps reinforce your nighttime cues, and over time, your circadian rhythm starts to follow the pattern.
The light–dark cycle is the most ancient and reliable signal life on Earth has ever known. It’s been a near constant throughout the entire course of evolution, shaping the biology of every organism that has ever existed. So, it’s no surprise it still shapes us today.
Dr. Jonathan Moustakis
Co-founder and CTO of Lume Health
References
Arendt J, Aulinas A. (2022) Physiology of the Pineal Gland and Melatonin.
https://www.ncbi.nlm.nih.gov/books/NBK550972/Borbély A. (2022). The two-process model of sleep regulation: Beginnings and outlook. Journal of sleep research, 31(4), e13598. https://doi.org/10.1111/jsr.13598
