Light on Every Floor

4. Screen frequencies & refresh rate

1. Neurobiological foundations: the non-visual pathway

From Keeler (1927) to CIE S026 standardization (2018)

Clyde Keeler observes that "apparently blind" mice retain a pupillary light reflex. He predicts the existence of a second photoreceptor system, independent of rods and cones. This intuition would take nearly 80 years to be fully confirmed.

. These cells, which represent only about 1% of ganglion cells in humans, project directly to the suprachiasmatic nuclei (

Melanopsin exhibits a peak sensitivity at 480 nm (blue-cyan), distinct from the cone peak (555 nm) and rod peak (507 nm). Melatonin suppression studies (Brainard et al., 2001; Thapan et al., 2001) established the action spectrum of this non-visual system.

ipRGCs encode ambient illuminance intensity, unlike rods and cones which encode contrast.

Melanopsin (OPN4) is a photopigment discovered in 1998-2002 in ipRGCs. Unlike rods and cones which encode contrast and color, melanopsin encodes ambient light intensity and projects to the suprachiasmatic nucleus (SCN), regulating circadian rhythms, melatonin secretion, wakefulness, and mood.

The International Commission on Illumination has established a standard metric to quantify the non-visual impact of light. The five α-opic receptors are now standardized: Melanopsin (ipRGC): α = 1.0 at 480 nm, M-cones: peak at 535 nm, L-cones: peak at 565 nm.

2. Attentional modulation in school environments

Controlled studies on lighting impact (2005-2023)

From the 2000s onwards, studies examined the impact of lighting on student cognitive performance. A study conducted in German schools compared rooms equipped with standard fluorescent tubes versus spectrum-controlled systems:

18% improvement in concentration scores under blue-rich morning lighting

Normalization of circadian rhythms (measured by actigraphy)

A systematic review published in Clocks & Sleep confirms that exposure to blue light (460-480 nm) in the morning improves wakefulness and cognitive performance, while evening exposure delays sleep onset and impairs sleep quality.

The terms "human-centric lighting" and "circadian lighting" designate installations whose spectra are selected according to the desired physiological effects (vigilance, melatonin secretion, circadian synchronization). The benefits of spectrum-controlled lighting in schools are documented by several controlled studies. The current normative framework (CIE S026:2018) specifies the technical parameters but does not regulate the information of occupants on the nature of the spectrum deployed, which is currently the responsibility of local communication devices.

Recommendations for lighting in establishments open to the public have been published over the last decade. The terms "human-centric lighting" and "circadian lighting" designate lighting devices whose spectra are selected according to physiological modulation objectives documented in the scientific literature.

3. Gamma induction (40 Hz): from fundamental research to applications

GENUS (Gamma Entrainment Using Sensory Stimuli) and neuroprotection

oscillations (30-80 Hz) are crucial for sensory integration, working memory and attention. Sensory stimulation at 40 Hz (GENUS) induces neuronal entrainment with effects on:

Reduction of amyloid plaques (Alzheimer mouse models)

Morphological modification of microglia (transition to phagocytic state)

The amplitude of visual oscillations is influenced by the frequency, chromaticity, and luminance of the light stimulus.

GENUS, GAMMA ENTRAINMENT USING SENSORY STIMULI

The GENUS protocol (Iaccarino et al., 2016) uses light flashes at 40 Hz to induce gamma oscillations in the visual cortex. These oscillations propagate to the medial prefrontal cortex and hippocampus, reducing amyloid plaques by 50% in mouse models. Human clinical trials are underway (Cognito Therapeutics).

3.1 Screen frequencies and accidental induction

Modern screens use variable refresh rates (60 Hz, 120 Hz, 240 Hz). The relationship with brain bands is complex:

60 Hz: can generate EEG artifacts and harmonics (30 Hz, 15 Hz) by beating

120 Hz: can induce oscillations in the low beta/gamma band via sub-harmonics

PWM (200 Hz - 2 kHz): produces imperceptible micro-flashes but detected by ipRGCs

A University of Michigan study (2016) shows that flicker light at 1 kHz can still induce a pupillary response via melanopsin, far beyond the visual fusion frequency.

4. Screen frequencies and parallel induction technologies

From 30 Hz television to 480 Hz screens: evolution and implications

50/60 Hz (field), 25/30 Hz (frame)

Most LED lighting and screens use pulse-width modulation (PWM) to control brightness. Although the switching frequency is generally >200 Hz (beyond conscious perception), ipRGCs exhibit an integrative response over longer durations. A light pulsed at 1 kHz can produce melanopsic activation equivalent to continuous light of lower intensity.

The PWM modulation of the screens produces micro-flashes at 200 Hz - 2 kHz, below the threshold of conscious perception but within the response band of the ipRGCs. This modulation is likely to affect circadian rhythms, melatonin secretion and arousal tone independently of conscious perception. It constitutes a neural training mechanism not perceptible at the population level, which has not, to date, been the subject of a systematic evaluation in public health.

The spectral and temporal properties of LED sources enable precise control of non-visual responses. The integration of specific flicker sequences in connected luminaires opens the way to "wellness programming" applications in controlled environments.

5. Incandescent to LED transition: neurochemical implications

Emission spectrum, melatonin suppression, and circadian modulation

Incandescence produces a continuous spectrum, close to blackbody radiation (2856 K), rich in red, poor in blue. White LED, on the other hand, emits a blue peak at 440-460 nm combined with yellow phosphor, producing an intense peak at 450 nm, exactly in the zone of maximum melanopsin sensitivity.

Evening exposure to white LED suppresses melatonin by 50% compared to 20% for incandescence at the same intensity.

EU 2009, USA 2012, China 2016: the gradual exit from incandescence has replaced the light spectra that have prevailed for a century with LED spectra whose energy distribution differs significantly, particularly in the 460 to 490 nm band used by ipRGCs. This transition, mainly motivated by energy efficiency, predates the consolidation of scientific knowledge on the melanopsin pathway (formalized by the CIE S026:2018 standard). Retrospective evaluation of its population neurochemical impact remains an active field of research.

EU 2009, USA 2012, China 2016. Since the gradual ban on incandescent lamps, the world population has been exposed to an artificial light spectrum radically different from what had prevailed for a century.

6. Optogenetics and targeted induction by LED flashes

Precise control of neuronal activity by light stimulation

uses light-sensitive proteins to selectively activate or inhibit neuronal populations. Channelrhodopsins (ChR2) are activated by blue light (~470 nm), halorhodopsins by yellow light (~570 nm).

Optogenetic stimulation of parvalbumin (PV+) interneurons at 40 Hz induces gamma oscillations and produces neuroprotective effects in models of neurodegenerative diseases.

Optogenetics allows millisecond-near control of specific neuronal populations through light stimulation. Human clinical applications remain limited due to the need for viral expression of heterologous opsins. Consumer LED technologies operate in the same wavelengths, without the cellular specificity that this expression confers. The analogy therefore remains strictly limited to spectral and temporal parameters, excluding neuronal selectivity.

6b. Individual variations: the PER2 factor

A 2019 study identified variations in the

(Period Circadian Regulator 2) gene associated with sensitivity to melatonin suppression by light. Three common haplotypes cover more than 96% of the chromosomes examined.

: homozygotes show significantly weaker melatonin suppression (p< 0.05)

, suggesting adaptation to high-latitude light environments.

The population is not uniformly sensitive to light interventions

PER2 gene variations demonstrate that sensitivity to light modulation is not uniform in the population. The ancestral haplotype, more frequent in Africa, confers relative resistance to melatonin suppression by blue light. Derived haplotypes, dominant outside Africa, render Eurasian populations more sensitive to blue-rich LED spectra.

The discovery of ipRGCs and melanopsin establishes a direct communication channel between the light environment and subcortical regions regulating wakefulness, mood, and circadian rhythms.

40 Hz stimulation (GENUS) modifies synaptic plasticity, reduces amyloid pathology, and transforms microglial morphology. These effects are reproducible and frequency-specific.

The LED transition modified chronic exposure

The near-total replacement of incandescent sources by blue-peak LEDs (450 nm) profoundly altered the nighttime light landscape, with measurable consequences for melatonin suppression at the population level.

Genetic variations modulate individual sensitivity

PER2 polymorphism demonstrates that the population is not homogeneous with respect to these interventions, which complicates any universal approach but also enables potential targeting.

International standards now frame measurement of these effects

Publication of the CIE S026:2018 standard formalizes institutional recognition of these phenomena and provides a common language to quantify non-visual impact.

The accumulation of scientific data on the mechanisms of neuronal induction by light and visual frequencies has enabled the transition from fundamental research to industrial applications and institutional recommendations (CIE S026:2018, successive sectoral standards). The dissemination of this knowledge to the general public, however, presents a time lag with its normative integration, which constitutes an area of possible improvement for public health and user information policies.