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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.

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

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 app tracks 11 phases. Each phase is a stretch of solar elevation, and the target warmth glides smoothly inside it. These are the values with the default settings (6500K daytime ceiling, 1800K night floor, both adjustable), for a New York style day:

• Deep night (below -18 degrees), Full darkness. 1800K.
• Early dawn (-18 to -12 degrees, sun rising), First hint of light. About 1800K rising to 2150K.
• Dawn (-12 to -6 degrees, rising), Horizon visible. About 2150K rising to 2500K.
• Civil twilight (-6 to 0 degrees, rising), Functional outdoor light. About 2500K rising to 3500K.
• Morning golden hour (0 to 10 degrees, rising), Warm sunlight. About 3500K rising to 6500K, or higher sooner with Morning Boost on (see below).
• Morning (10 to 30 degrees, rising), Bright morning. 6500K easing to about 6000K.
• Midday (above 30 degrees, rising), Peak daylight. About 5800K.
• Afternoon (sun descending to 10 degrees), Sun past its peak. About 5800K while the sun is above 30 degrees, then about 4900K easing to 4500K below it.
• Evening golden hour (10 to 0 degrees, descending), Warm sunlight. About 4500K falling to 3500K. There is no morning boost in the evening.
• Evening twilight (0 to -6 degrees, descending), About 3500K falling to 2500K.
• Night (below -6 degrees, descending), About 2500K falling to 1800K.

Morning and evening golden hour are not mirror images. Evening light has a stronger circadian impact, so in the evening the screen only follows the warm curve above. In the morning, Morning Boost (on by default, and only when your wake time is within 90 minutes of civil dawn) blends the screen toward your daytime setting. It starts at civil twilight and reaches full 6500K by the time the sun is 10 degrees up. That is why the app can read about 6400K in morning golden hour with the sun at 6.7 degrees, while the same phase in the evening sits near 3500K to 4500K. In the last two hours before your bedtime, the screen also eases to 1800K and dims slightly, whatever the sun is doing.

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 to 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 to 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

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 ramps the screen toward 6500K daylight-equivalent output from civil dawn, reaching full daylight white once the sun is 10 degrees up, to help set your clock.