Science

Built on real research

CircadianShield is grounded in peer-reviewed circadian photobiology. Here is the science that drives every decision.

Your Circadian Rhythm: The Master Clock

Your body runs on an internal clock called the circadian rhythm — a roughly 24-hour cycle that regulates sleep, hormone release, body temperature, and metabolism. The master pacemaker is a tiny region in the brain called the suprachiasmatic nucleus (SCN), located in the hypothalamus just above the optic chiasm.

The SCN needs external cues (called zeitgebers) to stay synchronized with the solar day. The most powerful zeitgeber is light. Specifically, it is light entering the eye and striking a specialized class of retinal cells called intrinsically photosensitive retinal ganglion cells (ipRGCs).

These ipRGCs contain a photopigment called melanopsin, which is most sensitive to short-wavelength light peaking around 480 nm (blue). When melanopsin absorbs this light, it sends a direct signal to the SCN: "It is daytime." When this signal arrives at the wrong time — say, from a laptop screen at 11 PM — the SCN gets confused, and your circadian clock shifts.

Melanopic EDI and CIE S 026:2018

Not all "blue light" is equal in circadian impact. The traditional approach of measuring illuminance in lux captures light as the human eye perceives brightness (via the photopic luminosity function). But lux tells you nothing about how light affects the circadian system.

In 2018, the International Commission on Illumination (CIE) published the CIE S 026:2018 standard, which defines a new metric: melanopic equivalent daylight illuminance (melanopic EDI). This metric weights the spectral power distribution of a light source according to the melanopsin absorption curve, giving a direct measure of circadian stimulus.

Melanopic EDI is the gold standard for predicting how a light source will affect the human circadian system. CircadianShield uses this metric — not just "color temperature" — to determine the biological impact of your display's output.

CIE S 026/E:2018 — CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light

Traditional "blue light filters" reduce blue wavelengths indiscriminately. CircadianShield's approach is more precise: it adjusts the display's spectral output to minimize melanopic EDI during evening hours while maintaining adequate melanopic stimulation during the day.

Blue Light and Melatonin Suppression

Melatonin is the hormone that signals your body to prepare for sleep. Its production begins in the evening (dim light melatonin onset, or DLMO) and peaks in the middle of the night. Exposure to blue-enriched light in the evening suppresses melatonin production, delays sleep onset, and reduces sleep quality.

A landmark 2014 study by Chang et al. found that reading on a light-emitting screen before bed (compared to a printed book) suppressed melatonin secretion, delayed the circadian clock by over 1.5 hours, and reduced next-morning alertness.

Chang A-M, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS. 2015;112(4):1232-1237.

A 2019 meta-analysis by Tähkämö et al. confirmed that evening exposure to displays with high blue content (400–500 nm) significantly delays melatonin onset and reduces total melatonin production. The effect is dose-dependent: brighter screens and longer exposure cause greater suppression.

Even moderate screen use (2 hours) in the evening produces measurable melatonin suppression. The suppression scales with melanopic illuminance — which is why CircadianShield reduces melanopic EDI rather than simply tinting the screen orange.

Tähkämö L, Partonen T, Pesonen A-K. Systematic review of light exposure impact on human circadian rhythm. Chronobiol Int. 2019;36(2):151-170.

Solar Elevation to Color Temperature

CircadianShield does not use a fixed schedule or a simple sunrise/sunset timer. Instead, it calculates the sun's elevation angle in real time using your latitude, longitude, date, and time. This elevation is then mapped to a target color temperature via a sigmoid transfer function.

The sun's journey through the sky defines natural light phases that humans evolved with:

  • Night (below -18 degrees) — Full darkness. 1800K deep amber.
  • Astronomical twilight (-18 to -12 degrees) — First hint of light. 2000K.
  • Nautical twilight (-12 to -6 degrees) — Horizon visible. 2400K.
  • Civil twilight (-6 to 0 degrees) — Functional outdoor light. 3200K.
  • Golden hour (0 to 6 degrees) — Warm sunlight. 4200K.
  • Low sun (6 to 15 degrees) — Bright morning. 5200K.
  • Full daylight (above 15 degrees) — Peak daylight. 6500K.

Transitions between phases use a smooth sigmoid curve to avoid any perceptible "jump" in color. The result is a display that changes as gradually and naturally as the sky itself.

The Phase Response Curve

Light does not affect the circadian clock uniformly across the day. The phase response curve (PRC) describes when light exposure advances, delays, or has no effect on the circadian clock:

  • Morning light (after your minimum core body temperature, typically 1–2 hours before habitual wake time) — Advances the clock. Makes you sleepy earlier the next evening.
  • Midday light — Minimal phase-shifting effect. Helps maintain stable timing.
  • Evening light — Delays the clock. Pushes your sleep time later.
  • Night light (especially first half of biological night) — Strong delay. This is the danger zone for screen use.

This is why CircadianShield's filtering is aggressive in the evening and night, moderate during the day, and why the Morning Boost feature delivers more blue light, not less.

Why Morning Light Matters

While most "blue light" products focus exclusively on blocking light at night, circadian health is a two-sided equation. Morning light exposure is equally critical.

Bright, blue-enriched light in the first 1–2 hours after waking:

  • Suppresses residual melatonin from the night
  • Triggers the cortisol awakening response (CAR)
  • Anchors the circadian clock to the solar day
  • Improves mood, alertness, and cognitive performance throughout the day

Lack of morning light is as disruptive as too much evening light. People who receive less than 30 minutes of bright light exposure before noon have delayed circadian timing and report poorer sleep quality.

Roenneberg T, Merrow M. The Circadian Clock and Human Health. Curr Biol. 2016;26(10):R432-R443.

This is why CircadianShield includes a Morning Boost feature. If you wake up to a dim room and immediately start working on a filtered, warm-tinted screen, you miss the morning light signal your SCN needs. Morning Boost temporarily delivers 6500K daylight-equivalent output during civil dawn to help set your clock.

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