Light and the body

Light at the eye

Light information enters the system via de eyes. The photopigments; light sensitive molecules located within the photoreceptors in the retina, absorb this light. Once light is absorbed, physical-chemical changes that affect the conformation of the photoreceptor, a process known as phototransduction takes place, which will ultimately evoke physiological responses

Rods and cones are the classical photoreceptors and their photopigments are rhodopsin and photopsin. Once light is absorbed the molecules switch to a high-energy state. During this process, the molecules fall apart and new ones have to be constructed in order to replace those that have been used. This regeneration process can occur only in the absence of light.

Some years ago, a new photoreceptor key for the circadian system has been discovered. The photosensitive retinal ganglion cells (pRGCs) containing the melanopsin photopigment (Provencio et al. 2000; Hattar et al. 2002, Berson et al 2002). Melanopsin does not have to be regenerated as the rhodopsin molecules, but shows instead two stable states (Melyan et al, 2005; Mure et al 2007) that exist in equilibrium under broadband light conditions. Exposure to cooler or warmer light (monochromatic short wavelength vs long wavelengths) can initiate a phototransduction cascade between the two states (Mure et al, 2007).

The spectral sensitivity of the phototransduction depends on the absorption characteristics of the photopigments. In humans, rods show a peak at around 498 nm, cones peak at 437 nm (short wavelength cones), 533 nm (medium wavelength cones), and at 564 nm (long wavelength cones) and the pRGCs peak at around 480 nm.

 

Extra-retinal light (the ear canal)

Recently, in 2010 a new light therapy device; the bright light headset (BLH), was introduced. This device allows for (extra ocular) transcranial light therapy via the ear canal. The idea of extra ocular light being capable of eliciting non-image forming responses is not new but is still under debate (see reviews Oren, 1996, Oren 2013). Evidence that this particular therapy works is however not strong and varies largely depending on the output being investigated. For instance, some recent studies support the hypothesis that transcranial bright light via the ear canal can affect psychomotor speed (Tulppo et al. 2014), reduce some related jet-lag symptoms (Jurvelin et al., 2015) and affect mood (Timonen et al., 2012; Jurvelin et al., 2013), though not always in the desired manner  (Pallesen et al, 2016). On the other hand, the same device was not capable of suppressing melatonin (Bromundt et al., 2014, Jurvelin et al. 2014) nor to alter sleep-wake parameters in phase delayed subjects (Pallesen et al, 2016).

 

Extra-retinal light (the skin)

Light in the eyes is important for vision and for the non-image forming effects such as synchronizing the sleep-wake cycle and improving alertness and performance. In 1998 a spectacular study was published saying that light at the back of the knees was effective in shifting the biological clock (Campbell & Murphy 1998). Since then, many groups tried to reproduce this effect of extraretinal light in humans, on the clock, on melatonin suppression, and on mood, but no one could find any support for the idea that light through the skin was capable of influencing the biological clock, nor inhibit melatonin production, nor affect mood (Koorengevel et al 2001, Rüger et al 2003, Wright & Czeisler 2002).

Nevertheless, light on the skin is important for other processes. In this case it is not the visual part of the light spectrum, but the shorter wavelengths that play an important role. In the skin, solar UV-B radiation drives the formation of pre-vitamin D3 which changes to vitamin D. While the summer months provide enough light for adequate levels of vitamin D in fair skinned people, the winter months often do not in Northern and Northwestern Europe.

Adequate levels of vitamin D are important for strong bones; vitamin D deficiency causes rachitis in children and severe deficiencies in adults lead to a softening of bones, muscular weakness and muscle cramps. Adequate levels of vitamin D have also been suggested to be important to decrease the risk of various diseases and conditions such as schizophrenia, autism, multiple sclerosis, diabetes, respiratory tract infections, influenza, and certain types of cancer, but adequate experiments indicating a causal relationship are sparse. Nevertheless it is recommended that a deficit in Vitamin D is compensated whether by means of supplements or diet (Health council of the Netherlands, U-7310/RW/db/877-D).

Regular indoor lighting will commonly not provide enough UV emission to be effective for vitamin D production. In some northern countries, vitamin D is added to some food (e.g. milk, margarine). Regular brief exposure in summer clothing to midday summer sun is enough to reach healthy levels of vitamin D. It is important to notice that prolonged UV exposure does not raise the levels of vitamin D3 but can instead increase the risk of undesirable effects such as the development of skin cancer (Scientific committee on emerging and newly identified health risks, SCHENIHR report, 2012). More information on sun light exposure and vitamin D supplementation can be found in a recent report of the Dutch Health council (https://www.gezondheidsraad.nl/sites/default/files/201215EEvaluationDietaryReferenceVitaminD.pdf).

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