Light and the human biology

Light is the most important “zeitgeber” to control our circadian rhythm. Daylight is therefore an important contribution to our wellbeing. Cool white wavelengths in the light supresses melatonin secretion.

The human visual and hormonal system. Light enters the eye and signals are sent from the retina to the brain's visual centre and suprachiasmatic nucleus. The most efficient light exposure angle for maximum ganglion cell triggering is from above the horizon. Source:

Scientists have been studying the biological impacts of light for decades. But it was not until 2002 that they discovered ganglion cells in the retina that are not used for seeing. The newly identified cells respond most sensitively to visible blue light and set the biological clock that synchronises our bodies with the external cycle of day and night.

The human eye retina contains three photoreceptors: colour sensitive cones, dim light sensitive rods, and blue light sensitive ganglion cells.

A major output of the biological clock system is the production of the hormone melatonin – a “sleep hormone”. This production in the pineal gland varies with the time of day. Melatonin is secreted at night and has minimal levels during daytime. A larger melatonin suppression, triggered by light exposure, often coincides with increased feelings of alertness and higher sustained attention.

The hormonal pulse generators
The ganglion cells send signals to the brain and regulate hormone production. The three most important hormones that control the biological rhythm are:
1. Melatonin makes you tired, slows the body functions and lowers activity in favour or earned rest.
2. Cortisol on the other hand is a stress hormone that is produced from about 3 am. It stimulates the metabolism and programs the body for day mode.
3. Serotonin works as a stimulant and motivator. While the cortisol level in the blood drops throughout the day and thereby acts counter-cyclic to the melatonin level, serotonin helps elevate energy levels.

Implementing Human Centric Lighting 

In order to install and program efficient Human Centric Lighting solution, four parameters require careful attention: spectrum, intensity, timing & duration and distribution Each parameter may be changed as long as one or more of the other parameters are adjusted accordingly. 



Timing and duration 



Light is the radiation visible to the human eye in the 380–780 nanometre range. Optical stimuli are registered in the human eye by three different cones which respond sensitively to red, green or blue radiation. But we do not perceive colours as equally bright. Colours in the yellow-green spectrum at 555 nanometres are perceived as the brightest. The rod cells enable us to see in dim light. They cannot distinguish colours, however. The biologically effective range is the blue spectrum around 460 nanometres.

The sensitivity curves under daylighting conditions v(λ), nighttime v'(λ) and for circadian effects c(λ)

The ganglion cells are most sensitive to light at 480 nanometres (1). This corresponds to blue light. The equivalent white light will therefore contain a large portion of blue wavelengths and is hence referred to as cool white light, with colour temperatures from 5-6000 kelvin and above. Research (2) has shown that exposure to light in the blue part of the spectrum results in lower melatonin secretion. In short, we could say that the cool white light that we find much of in sunlight and certain light sources will help adjusting the circadian phase and result in a higher subjective alertness, core body temperature and heart rate (3).

The spectral distributions of various light sources. Cool white LED light has a higher amount of blue wavelengths, and is therefore more effective when it comes to adjusting circadian rhythms.

1 Bailes, H.J. and Lucas, R.J. (2013) Human melanopsin forms a pigment maximally sensitive to blue light (lmax _479 nm) supporting activation of Gq/11 and Gi/o signalling cascades. Proc. Biol. Sci. 280, 20122987
2 Brainard et al., 2001 Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. The Journal of Neuroscience, 21, 6405-6412.; Thapan et al., 2001 An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans. The Journal of Physiology, 535, 261-267.
3 Cajohen et al., 2005 High sensitivity of human melatonin, alertness, thermoregulation, and heart rate to short wavelength light. The Journal of Clinical Endocrinology & Metabolism, 90, 1311-1316.


The melatonin suppression starts at 30 lux and saturates at approximately 1000 lux at eye level. Knowing that melatonin levels saturate above 1000 lux at the eye could be used as a maximum level guideline. This translates to a vertical illumination, or cylindrical illuminance Ez, of 1000 lux. (Elderly people with reduced vision will need a higher illumination level). In 2019 Underwriters laboratories (UL) presented new recommendations for lux level in order to achieve melatonin suppression. The recommendation is for 254 lux on the eye (measured vertically) provided the use of indirect light and a colour temperature of 500 Kelvin. If the colour temperature or light distribution is altered, the recommended lux level will also change. Glamox uses this as a basis when we customise our Human Centric Lighting solutions.

The Underwriters laboratories (UL) recommends 254 lux at eye level provided the use of indirect light and a colour temperature of 500 Kelvin.

Because of physical laws, the horizontal illuminance on the work surface (at 0,75 m above floor level) will be 2 or even 3 times higher than at eye level. That may again pose great challenges in terms of glare and energy consumption. Our recommendation is therefore to reduce the light level to maximum 250-350 lux at eye level (corresponds to approx. 750-1000 lux at the work plane) and rather prolong the exposure time. This does not necessarily reduce the energy consumption, but will benefit the lighting conditions.

What is cylindrical illuminance?
EN 12464-1 calls for more light on people’s faces to improve conditions for visual communication. In areas, where good visual communication is important, especially in offices, meeting and teaching areas, Ez should be no less than 150 lx with U0 ≥ 0,10. Picturing peoples’ head as cylinders, the cylindrical illuminance is the average of all light (measured in lux) that falls on the cylinder.

Cylindrical illuminance Ez is the average of all vertical light that hits an imaginary cylinder.

Maintenance factor
The lamp lumen maintenance factor (LLMF) for Human Centric Lighting solutions should be kept at 1,0. This is because a lighting installation that is dimensioned to an Ez of 250-300 lux will have enough light to fulfil requirements for both visual task lighting and the wanted biological effects. With time, the lumen output will diminish, but it will still be sufficient to do the visual tasks. The consequence is, however, that the duration of the circadian effective light exposure must be prolonged to obtain the same effects as in the beginning. Since no clear guidelines exist on lumen maintenance levels or duration periods, we recommend that the LLMF is set as high as possible.

Ez is perhaps not the most accurate proxy for illuminance at the eye level, where we want the light to hit, but it is a pragmatic approach with many advantages. Number one being that this is a metric that the lighting planner already understands and uses. The relationship between Ez measured or calculated in an activity plane 1,2 m above the floor for sitting people and the task area illuminance Em at 0,75 m is between 1:2 or 1:3. Hence, light at the eye is in most cases lower than light at the work surface.

M. Gibbsa,b, S. Hamptona, L. Morganb, J. Arendta, 2002. Adaptation of the circadian rhythm of 6-sulphatoxymelatonin to a shift schedule of seven nights followed by seven days in offshore oil installation workers. 
2 Smith, Revell & Eastman, 2009; Smith and Eastman, 2009 Phase advancing the human circadian clock with blue-enriched polychromatic light.

Timing and duration

Non-visual effects of light are influenced by the time of day. 

Light in the morning is the most effective. It tells our biological clock that the day has begun and that bodily functions need to be activated. Conversely, light exposure in the evening will cause melatonin production to be suppressed and make it harder to fall asleep. Exposure at night, before the point that the core body temperature reaches its minimum (nadir) can result in a phase delay, while exposure in the early morning (after nadir) can cause a phase advance. The acute effects on alertness, however, do not depend on time of day. Effects on sustained attention are significant only in the morning (1).

Human psychology also plays a role with respect to timing of colour temperature variations. Preferred light settings may vary with time of day. Therefore, users should be given the opportunity to adjust colours themselves, preferably when the risk of phase delay or advances is lower.

A general rule of thumb is that the larger the time of exposure the larger the phase shift (2). But this relationship is not necessarily linear. It may be that people are more sensitive to light in the first part of the light exposure (3). Short durations of bright light exposure may also induce phase-shifts in circadian rhythms. Instant effects of bright light on subjective alertness, however, may not be dependent on the duration of exposure. Instead, continuous or repeated exposure is required when activation is intended (4).

Therefore, it is difficult to state clear guidelines with respect to duration. A compromise needs to be made between personal preferences, the wanted phase-shifting effect and energy consumption. A working hypothesis for our Human Centric Lighting installations is to provide phase-advancing, blue-enriched bright light in the late morning to allow for night owls to pass their core body temperature minimum. And we recommend giving users access to alertness-enhancing light during the workday, with moderate duration times. This could be implemented by combining a pre-programmed lighting cycle with individual control over colour temperature and dim levels.

1 Smolders et al.2012 A higher illuminance induces alertness even during office hours: findings on subjective measures, task performance and heart rate measures. Physiology & Behavior, 107, 7-16.
2 Chang et al., 2012 Human responses to bright light of different durations. Journal of Physiology, 590, 3102-3112.; Dewan et al., 2011 Light-induced changes of the circadian clock of humans: Increasing duration is more effective than increasing light intensity. Sleep, 34, 593-599.
3 St.Hilaire et al., 2012 Human phase response curve to a 1h pulse of bright white light. Journal of Physiology, 590, 3035-3045 and Rimmer et al., 2000 Dynamic resetting of the human circadian pacemaker by intermittent bright light. American Journal of Physiology - Regulatory Integrative and Comparative Physiology, 279, 1574-1579.
4 Vandewalle et al., 2009 Light as a modulator of cognitive brain function. Trends in Cognitive Sciences, 13, 429-438.

Learn more about the timing and duration of light

The time of day for the exposure to cool white or warm white light is important because of its effect on the daily rhythm. To understand this, we have to examine the human performance curve. We call the point when hormone production and body temperature is at the bottom“nadir”. This normally happens two hours before natural wake-up time. So, if your natural wake-up time is 7 am, nadir is at 5 am. Light exposure before nadir will shift the daily rhythm forward, whereas exposure after nadir will shift it backwards.

Human performance curve over the day: body and mind are fittest around 10 a.m. Two hours before wake-up time, they reach a low. A natural dip also occurs in the early afternoon. Light exposure before nadir phase-shifts the daily rhythm backwards, whereas light after nadir has a phase-advancing effect.

The human performance curve is different for different chronotypes. Hence the ideal time for exposure to circadian effective light is also different.

Human psychology also plays a role with respect to timing of colour temperature variations. Preferred light settings may vary with time of day. Therefore, users should be given the opportunity to adjust colours themselves, preferably when the risk of phase delay or advances is lower.

We also know that light in the early hours after nadir gives stronger effect on rhythm shifts than later in the day. Knowing this, we can set the time for light exposure to achieve the wanted effect. But since people have different chronotypes, the timing has to be set carefully. ”Night owls” have a later wake-up time than early risers or “larks”. The duration of the cycle may also vary from 23 hours for the lark to perhaps 26 hours for the owl. Hence, one may risk feeding the owls with activating light at the wrong side of nadir. For normal office hours, light exposure from 9 am and onwards could therefore be used as a rule of thumb. This will help people to adapt better to less daylight in the winter time.

For workers on night shift over several days, it might be helpful to shift the rhythm 8 hours backward to reduce the sleepiness during night. Using the knowledge above, this can be achieved by exposing the worker to cool white light in the late evening/early night for a couple of hours (depending on the intensity). This will shift the curve backwards over a couple of days.

Cool white light may have a positive effect on subjective alertness and mood. The effect is sudden and will pass minutes after the exposure has ended. These acute effects of bright light exposure on subjective alertness, fatigue and vitality are also independent on time of day. However, the effect of bright light exposure on sustained attention is most significant in the morning.

What about individual differences and preferences? 
Not only human chronotypes vary, but also their preferences. There is no one size fits all for tuneable white lighting solutions. What may have excellent phase-shifting effects on one person may have negative effects on others. In class rooms, where the level of individual adaptation of the lighting is more difficult to set, pupils and teachers will receive the same spectrum, intensity and exposure. This calls for even greater care to be taken when planning for these applications. The same applies for open offices, but individual adaptations are still easier to implement in the form of personalised lighting such as task lights, free standing lights or pendants above each work station. Lighting solutions in industrial work stations and patient rooms in hospitals may also be easier to set individually.

Distribution of light 

The ganglion cells of the third photoreceptor are most sensitive in the nasal and lower area of the retina. This is due to the eye adapting to natural lighting conditions, because daylight enters the eye from above. 

Light coming from angles above 60 ° relative to the horizontal plane and below the horizontal plane has little or no effect on melatonin production. This since most Ganglion cells are placed in the nasal and lower area of the retina. Light coming from the "right" angle must not be perceived as discomfort glare.