Light-dark adaptation- and the sunshine effect that keeps on giving

Critical contrast 

Your body clock evolved to respond to regular patterns of bright light and darkness.

If you get enough bright light in the morning, your body clock will stay on track even if you get too much light in the evening, although low light in the evening is best – Blue-Enriched Morning Light as a Countermeasure to Light at the Wrong Time: Effects on Cognition, Sleepiness, Sleep, and Circadian Phase.

Dynamic lighting seems to improve sleep at night for office workers who get at least a few moments of daylight on their commute – Temporal tuning of illuminance and spectrum: Effect of a full-day dynamic lighting pattern on well-being, performance and sleep in simulated office environment.

Delivering a clear pattern of bright days and dark nights may even boost sleep and mood in people working shifts in closed environments – Active interventions of dynamic lighting on human circadian rhythm and sleep quality in confined spaces.

But the effects of the ‘diet of light’ over the day on sleep, mood and memory are especially marked in people living with dementia with reduced hormone signalling due to changes in the neural networks that regulate the body clock – The aging clock: circadian rhythms and later life, and who often do not have the luxury of time outside – Bright Light Delights: Effects of Daily Light Exposure on Emotions, Restactivity Cycles, Sleep and Melatonin Secretion in Severely Demented Patients.

Blinded by the Light: The Challenges of Light-Dark Adaptation

As the days get longer and brighter, you may have found yourself hesitating on the threshold of a building, disoriented by the sudden shift in light levels.

This momentary confusion is uncomfortable if you’re steady on your feet and simply walking into a store or out into a familiar street.

But uneven lighting is a major contributor to trips and falls on construction sites and for the elderly, often with devastating consequences: falls are estimated to be the highest cause of non-self-inflicted causes of fatal injuries among older people (60+ years) in the EU – Non-motor Pedestrian Accidents: A Hidden Issue.

Luckily, it’s relatively easy to fix: one study by the Centre for Disease Control and Protection found that improved lighting improved trip hazard detection by over 48%, even in the punishing conditions of a mine – Mining Publication: LED Lighting for Improving Trip Object Detection for a Walk-thru Roof Bolter, 

So what is going on when you are forced to adapt to extreme shifts in light levels- and what can you do to ease the transition?

Your visual system evolved to thrive under natural lighting conditions.  As a result, our eyes respond most efficiently – and tend to prefer – scenes with similar levels of contrast (an even distribution of light levels known as the 1/fx curve), and divided mostly along horizontal and vertical lines – Differential Effects of Orientation and Spatial-Frequency Spectra on Visual Unpleasantness, The “Horizontal Effect”: A perceptual anisotropy in visual processing of naturalistic broadband stimuli.

There are two complementary systems at play:

Light adaptation – Going from dark to light. The muscles around the pupil constrict to close the aperture, reducing the amount of light getting into the eye. That can take anywhere between 0.18 to 2 seconds –Pupils. In the meantime, that blast of bright light will bleach out or break down the proteins in the rods and cones. So those cells stop sending signals to the brain and you are temporarily blinded. As these proteins refresh, you may notice patches of the opposite colour ‘floating’ in your visual field.

Dark adaptation – Going from light to dark. Low-light vision, or scotopic vision, is driven by the rods. Initial adaptation takes around 60 seconds for a healthy adult but full dark adaptation can take up to an hour. Difficulties with dark adaptation early sign of AMD – Dark Adaptation and Its Role in Age-Related Macular Degeneration. A new mobile app that tests your ability to adapt is 72% accurate in diagnosing age-related AMD, similar to tests carried out in a specialist clinic – Evaluation of a mobile app for dark adaptation measurement in individuals with age-related macular degeneration.

So what can you do?

1. Eat your greens.

Vitamin A can help to boost your ability to respond, especially for older adults – LOW-DOSE SUPPLEMENTATION WITH RETINOL IMPROVES RETINAL FUNCTION IN EYES WITH AGE-RELATED MACULAR DEGENERATION BUT WITHOUT RETICULAR PSEUDODRUSEN. Even low doses make a significant difference so consider a supplement or eating foods such as oily fish and leafy vegetables – Vitamin A and Carotenoids.

2. Smooth transitions.

Add a shading canopy above doorways if you can and brighter lights just inside the entrance to your office or home will ease the transition. Make sure the floor is level and even and make sure any rugs or mats are less than 10mm high, the critical height for trip hazards at night – Illuminance required to detect a pavement obstacle of critical size.

Avoid strong, focused light sources, high contrast graphic patterns and glossy surfaces in these transition zones because these can make the natural disorientation and confusion worse.

3. Consider light-sensitive glasses. 

There are two broad types:  photochromic and polarising lenses.

Photochromic, or light-adjusting lenses contain organic photochromic molecules that have been embedded in the layer near the surface of the lens. When UV light hits them, the molecules change shape and absorb the visible light, making the lenses darken. When UV light is absent the molecules can reverse to their original shape, thus providing a clearer lens.

These are different to polarised lenses. A photon or unit of light travels in a spiral or wave. When it hits a surface, the wave bounces away, and the spin is reversed, creating a chaotic or fragmented signal that the eye can perceive as glare. The polarised lense works like the slats of a Venetian blind, letting part of the wave through and blocking selected reflected waves – Polarization of Light.

The bright and the dark sides of the UV index

It’s finally sunny over here UK and the UV index is starting to climb.

What does that mean?

UV refers to a group wavelengths at the blue edge of the visible spectrum – between 100 and 400 nanometers.

It’s usually divided into three.

UVA (315–400 nm) – goes deepest into your skin

UVB (280–315 nm) – absorbed by the top layer of your skin and makes you burn

UVC (100–280 nm) – is mostly blocked by the earth’s atmosphere – SOLAR AND ULTRAVIOLET RADIATION.

Your body needs sunlight and particularly those UV wavelengths to generate Vitamin D for brain and bone health, boost mood, muscle recovery and immune response – Vitamin D-independent benefits of safe sunlight exposure, The effects of vitamin D on skeletal muscle strength, muscle mass, and muscle power: a systematic review and meta-analysis of randomized controlled trials.

Getting a burst of UVB can even help you sleep –The effect of narrow-band ultraviolet B radiation on sleep, happiness, and appetite: A prospective cohort study.

But of course, too much UV can harm your skin, leading to premature aging, painful sunburn and even cancer – How does the skin sense sun light? An integrative view of light sensing molecules.

The same goes for your eyes. Prolonged exposure is not just tiring it can increase the risk of cataracts and other eye conditions – Correlation of Sunlight Exposure and Different Morphological Types of Age-Related Cataract.

So, what can you do?

Embrace the sunshine – but treat it with respect.

Be aware that UV makes it through cloud cover – it can even make it stronger – and surfaces like sand and water bounce light back so you can get sunburn even if you’re sitting under an umbrella on the beach – SOLAR AND ULTRAVIOLET RADIATION.

Sunscreen, sunglasses and protective clothing are great but can give you a false sense of security – some of the most harmful rays might still be getting through.

So cover up and aim for short bursts, ideally early morning and late afternoon – around 1/3rd of the UV dose is delivered in those three intense hours between 11am and 1pm – Radiation: Ultraviolet (UV) radiation.

Time to head outside!

Personality in the Spotlight: How Light Shapes Behavior

Finally, as we see peak temperatures rise across the world, this paper shines a light on the links between climate and agresssion. It notes that extreme temperatures, both heat and cold, are associated with increased violence and antisocial behaviour, although these effects are moderated by rainfall, forests and proximity to a coastline – Thermal demands and its interactions with environmental factors account for national-level variation in aggression.

But natural light is a consistent predictor of positive social interactions and reduced disputes around the world, perhaps due to increased levels of serotonin production. 

It notes that this relationship is not a linear one: above a baseline, your response to bright light depends on your personality type: if you are strongly interdependent (you identify strongly with others in your community), more light increases your tendency to do the right thing. Equally, for extreme independent, being in the spotlight tends to increase negative, antisocial behaviour.

Another paper looking at our willingness to donate money and help others found similar results: interdependents under bright light donate more time and money to a good cause; while independents  donate less – Illuminating illumination: Understanding the influence of ambient lighting on prosocial behaviors.