Blue light, melatonin, and the menstrual cycle: why biological night matters

Not every dark night is biological darkness.

You can turn off the bedroom light, get into bed, sleep seven hours, and still not have given your body a complete endocrine night. The difference lies in a signal that almost never appears in conversations about hormonal health: melatonin.

Melatonin is not just "the sleep hormone". It is a darkness signal. It tells the body that night has arrived: it lowers alertness, helps organize temperature, speaks with the circadian clock, and may influence how the reproductive system interprets time.

That is why the question is not only "how many hours did I sleep?".

The more interesting question is: did my body receive a real biological night or only time in bed?

The screen is not just a screen

Light enters through the eyes, but its effect does not end with vision.

The retina has intrinsically photosensitive ganglion cells. These cells express melanopsin, a pigment especially sensitive to wavelengths near blue. When they receive enough nighttime light, they send a signal to the suprachiasmatic nucleus, the brain's master clock.

That clock regulates output to the pineal gland. In sufficient darkness, the pineal gland produces melatonin. With nighttime light, especially blue light or intense LED light, that production can decrease, be delayed, or become shorter.

This matters because two lights that "look" similar can have different biological power. Traditional lux measures brightness for human vision. Melanopic load better measures the circadian potential of that light.

In other words: the issue is not moralizing the phone. The issue is that a screen close to the face, with high brightness, in the hour before sleep, can act as a biological signal that says "it is still not night".

What the studies show

In 2025, Sanchez-Cano and colleagues compared red light and blue light during nighttime exposure in healthy adults. After two hours, blue light kept melatonin much lower than red light: 7.5 pg/mL versus 26.0 pg/mL.

Schollhorn and colleagues, in Communications Biology, showed that the melanopic irradiance of screens predicted longer sleep latency, lower melatonin, and delayed melatonin onset. The finding is important because it is not enough for a screen to look "warm" if its melanopic load remains high.

Gooley and colleagues, in a classic Journal of Clinical Endocrinology & Metabolism study, found that room light before bedtime delayed melatonin onset in almost all participants and shortened its duration by around 90 minutes.

That is what we call a compressed biological night: you do not necessarily sleep zero hours, but you lose part of the hormonal signal that tells the body that the night was night.

The key nuance: cycle does not mean identical sensitivity

There is a tempting but incorrect phrase: "in the luteal phase, blue light suppresses melatonin more".

It is better not to say that.

Vidafar et al. 2024 studied circadian sensitivity to light in women and men. They found greater melatonin suppression in women than in men under bright light, but they did not find differences by menstrual phase or an acute association with estradiol or progesterone.

So why does this matter for the cycle?

Because phase does not have to change melatonin sensitivity to light in order to change the cost of losing melatonin.

In the luteal phase, after ovulation, progesterone rises and basal body temperature usually increases. Many women already have more vulnerability to fragmented sleep, awakenings, irritability, cravings, or low energy before the period. If you add a biological night shortened by light to that context, the symptom may feel stronger even if the initial biological response to light is similar.

The careful mechanism is this:

• nighttime light can reduce or delay melatonin;
• melatonin helps organize the darkness signal, temperature, and sleep;
• the luteal phase already changes temperature, progesterone, and sensitivity to rest;
• that is why the same loss of darkness can feel different depending on the hormonal moment.

It is not that the screen directly "changes your cycle". It is that it can erode a signal the hormonal system uses to organize the night.

Perimenopause: when night becomes more fragile

For Carmen, 47, the problem may look different.

In perimenopause, ovulation can become intermittent, progesterone fluctuates, and vasomotor symptoms can appear before the cycle becomes clearly irregular. Many women describe waking with heat, sweating, palpitations, nighttime anxiety, or brain fog the next day.

In that context, low or delayed melatonin may not be "the cause" of the problem. But it can act as an amplifier.

If the night is already strained by temperature, estrogen changes, awakenings, or stress, a high light load before sleep can remove part of the signal that normally helps the body enter nighttime mode.

The question for Lua would not be:

"Does the screen cause perimenopause?"

It would be:

"Do nights with more screen use or intense light before sleep precede more awakenings, worse morning energy, or heat symptoms in this woman?"

That difference changes everything. One phrase is causal and simplistic. The other is longitudinal and measurable.

The double nighttime collision

At Lua Labs, we had already been investigating another piece: late dinner close to biological night.

Eating late can be a different metabolic load when melatonin is already high and the body is preparing for sleep. Blue light adds a second problem: it can delay that melatonin, push sleep later, and compress the endocrine night.

The combination could look like this:

1. screen or intense light in the hour before sleep;
2. late dinner, snack, or alcohol;
3. delayed or shortened melatonin;
4. temperature, glucose, alertness, and digestion competing with the beginning of the night;
5. worse energy on waking, non-restorative sleep, or more intense symptoms in the luteal phase.

This is a hypothesis, not a clinical recommendation. But it is a useful hypothesis because it can be observed within the same person.

We do not need to compare one woman with another. We can compare her own nights:

• low light and early dinner;
• high light and early dinner;
• low light and late dinner;
• high light and late dinner.

If the fourth group repeatedly comes with worse sleep, energy, or symptoms, a personalized clue appears.

What Lua could measure

Lua does not measure melatonin. It also does not measure melanopic lux with clinical precision.

But it can approximate the context with simple questions and cross it with longitudinal data.

The minimum question would be:

"In the hour before sleep, were you using a screen or exposed to intense light?"

Options:

• no;
• a little;
• yes, quite a lot.

With that, a proxy could be built: NML, Nocturnal Melanopic Load. It would not be diagnostic. It would be an estimate of nighttime light load relative to sleep.

Then Lua could cross it with:

• bedtime;
• awakenings;
• energy on waking;
• mood;
• luteal symptoms;
• late food;
• cycle phase;
• perimenopause;
• wearable data when available, such as HRV, RHR, temperature, or minutes awake.

The value is not in saying "the screen is bad". The value is in detecting whether, for a specific woman, nights with more light before sleep are associated with a worse hormonal-symptomatic pattern.

The answer is not to prohibit. It is to map.

Many health conversations turn chronobiology into guilt: do not use your phone, eat dinner early, sleep perfectly, do everything right.

That approach fails because not everyone lives the same life. One woman may work nights. Another may care for children. Another may study late. Another may have hot flashes that wake her up even when she does everything "right".

Lua does not need to moralize the night. It needs to map it.

The powerful question is not:

"Did you follow the rule?"

It is:

"What pattern repeats when your body cannot enter biological night?"

If a woman discovers that her worst energy days happen after intense screen use + late dinner + luteal phase, she has a more concrete conversation with herself and with her health professional. Not a label. Not a diagnosis. A pattern.

That is longitudinal hormonal intelligence.

Sources

• Hu J, Li S, Yu X, Dai L. From nighttime light exposure to menstrual health: a critical review of evidence, mechanisms, and nursing interventions. Frontiers in Reproductive Health. 2026.
• Vidafar P, McGlashan EM, Burns AC, et al. Greater sensitivity of the circadian system of women to bright light, but not dim-to-moderate light. Journal of Pineal Research. 2024.
• Sanchez-Cano A, Luesma-Bartolome MJ, Solanas E, Orduna-Hospital E. Comparative Effects of Red and Blue LED Light on Melatonin Levels During Three-Hour Exposure in Healthy Adults. Life. 2025.
• Schollhorn I, Stefani O, Lucas RJ, Spitschan M, Slawik HC, Cajochen C. Melanopic irradiance defines the impact of evening display light on sleep latency, melatonin and alertness. Communications Biology. 2023.
• Gooley JJ, Chamberlain K, Smith KA, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. Journal of Clinical Endocrinology & Metabolism. 2011.
• Brown TM, Brainard GC, Cajochen C, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology. 2022.