The extra-visual effects of light, such as a decrease in melatonin, a shift in circadian phase, or a change in circadian amplitude, are mediated by a lesser-known type of photoreceptor, the light-sensitive retinal ganglion cells (ipRGCs), which are adapted to outdoor light conditions and allow mammals to synchronize circadian rhythms to a 24-hour period of alternating day and night. Artificial lighting in buildings is, with few exceptions, below the threshold of effect in this respect. Moreover, the need for light increases with age. A weak light stimulus can lead to a loss of synchronisation. Properly timed (morning) light of the necessary intensity (thousands of lux) and appropriate spectral composition (cool tone) can supplement the missing light information and thus restore synchronisation. Therapeutically, bright light is used, for example, in the treatment of depression or seasonal affective disorder. As a measure within the built environment, bright light finds application in the health and social services.

INTRODUCTION

Light allows us to see, it goes hand in hand with heat, and it is the source of virtually all the energy in our diet. However, most discoveries in the extra-visual effects of light have been made in the last 40 years. In addition to rods and cones, another photoreceptor has been discovered in the retina: the light-sensitive ganglion cells (ipRGCs), and it has been shown that these neurons containing the photopigment melanopsin signal information about the intensity of ambient light to the suprachiasmatic nuclei (SCN) in the hypothalamus via the visual pathway tap (RHT). Here, in mammals - including humans - the body's central clock resides. The period of this gene oscillator would be approximately 24.2 hours in a young healthy individual without synchronization (previously reported to be 25.2 hours), but from the equator to the outer edges of the temperate zone, this oscillator is synchronized each morning by intense light to start the day. The sleep/wake rhythm and the local clocks of the various organs are then synchronized according to this central biological clock. Melatonin, cortisol or body temperature levels have their typical physiological patterns in this 24-hour (circadian) rhythm - for example, sleep hormone levels rise in the evening, are high at night and fall sharply in the morning. Similarly, there are periodic changes during the day in, for example, attention, reaction time or physical or mental performance. The light-synchronized rhythm of temporal genes is also the rhythm of human society.

EXTRA-VISUAL EFFECTS OF LIGHT

In the literature, we can also encounter the term circadian, melanopic or chronobiological or non-image forming effects of light. The light-sensitive ganglion cells of the retina are the most involved, but the influence of cones or rods cannot be neglected. These include the following mechanisms(1):

Decrease in melatonin at night: intense light at night results in a stop in the production of melatonin in the pineal gland and can cause insomnia. See below: light at night.

Circadian phase shift: with the help of a bright light, the phase of the circadian rhythm can be shifted forward or backward by a few hours. The dependence of the shift on the time of day of light application is expressed by the phase response curve(4) (PRC), see Figure 1.

Change in circadian amplitude: regular illumination with bright light in the morning hours can restore or normalize the course of the disturbed circadian rhythm.

Light activation: projecting bright light onto the retina rapidly activates attention and has a pro-cognitive effect(5).

Pupillary reflex: light-sensitive retinal ganglion cells form the slow component of the pupillary constriction reflex. Its changes in Alzheimer's patients could have diagnostic applications.

QUANTIFICATIONS

Light-sensitive ganglion cells are most sensitive to the blue component of light in the region around 480 nm. At this wavelength, for example, the light-emitting diodes of blue ambulance beacons glow. The proportion of the activating blue component for a given light is expressed by the indices Xmel,D65 (1) or Ac (2), which can be used to convert the easily measurable photopic ("visual") lux to the intensity of the extra-visual stimulus. For example, incandescent light contains only about a third of the blue component compared to daylight of the same intensity; the most common fluorescent lamps contain about half. With increasing age, a smaller percentage of incident light is projected onto the retina, and therefore the intensity needs to be corrected for age. According to (1) and (3), an approximate relationship for the correction factor can be derived in the form k = 100 / (120 - 3 / 4 V) where V is the age of the patient. Approximately twice the intensity is required at 90 than at 25. The mechanism of the effect of light on circadian rhythm starts to work from approximately 1000 lux illuminance at eye level (+ correction for age). In practice, intensities of 10 000, 5000 and 2500 lux are often used. The threshold dose for a reliable effect of light on circadian rhythm is 10 000 lx for 20-30 minutes. At lower (but above threshold) intensities, the exposure is proportionally longer.

The light-sensitive ganglion cells are distributed throughout the retina except in the blind and yellow spots. Most of them are found around the macula, while less concentrated cells on the periphery of the retina have more extensive light-sensitive processes. For stimulation, it is sufficient that the light source is in the field of view; it is not necessary to look directly into the light. However, it is necessary to keep the eyes open - only a fraction of a percent of the incident blue component passes through the closed eyelids.

Since the first experiments with light in seasonal affective disorder in the 1980s, full-spectrum fluorescent lamps of cool daylight tone with excellent colour rendering (Tc > 5500 K, Ra > 90) have been used. Such artificial light has the necessary proportion of the effective blue component and is also more user-friendly than cool light for general illumination.

LIGHT IN NATURE

The midday intensity of the horizontal component of daylight is approximately 100 000 lx in summer and only about 20 000 lx in winter. For an upright person, the vertical illuminance is of importance, as it is very directional to the position of the sun, includes a reflected component and can reach intensities of tens of thousands of lux even in winter. The sun rising on the horizon gives an illuminance of approximately 1000 lux under clear skies. Thus, under the open sky, the extra-visual photoreceptors are deep in the saturation zone for most of the day. The lower forest floor receives about 2% of the ambient illuminance, so here the photoreceptors are in saturation for only part of the day.

DAYLIGHT IN BUILDINGS

Modern society is characterised by a comfortable way of life, associated with the relocation of people from open space to buildings, where only a few percent of the sunlight intensity penetrates at a limited angle. Although relatively strict requirements are imposed on new construction in terms of minimum daylight factor, these are based on the quantity of light needed for visual tasks (hundreds of lux) and not for non-visual effects (thousands of lux). Thus, whether the required intensity is achieved depends mainly on the orientation of the room to the south, the size of the windows, the height of the sills and the distance of the observer from them, the degree of shading (curtains, drapes), the reflections in the interior and the surrounding buildings. In addition, daylight varies throughout the year; at the winter solstice the day is only 8 hours long, the sun is lowest in the sky and the light intensity is minimal. Therefore, daylight in buildings generally does not provide the necessary extra-visual stimulus. Moreover, many health and social care facilities are located in historic buildings surrounded by tall trees where the contribution of daylight is negligible and, without artificial lighting, we measure only a few tens of lux on patients' faces at midday, well below the circadian effect. The lack of light stimulus may then contribute to the disruption of the circadian rhythm.

ARTIFICIAL LIGHT IN BUILDINGS

When daylight is unavailable in time and climate, interiors must be illuminated with artificial light - most often electric. The requirements for artificial lighting are again based on the need for light for visual tasks - "to see to work and to travel". For example, for patient rooms, the standard(6) requires 100 lx at floor level and 300 lx for reading - sub-threshold values in terms of extra-visual effects, especially when using neutral or warm tones of light with a small activating blue component. The situation is better in staff offices, where the standard requires 500 lx, and in examination and treatment areas 1000 lx of maintained illuminance. The intensity of artificial lighting designed for visual tasks is therefore insufficient to activate extra-visual effects in most interiors.

LIGHT IN THE NIGHT

At low light levels at night, the non-visual mechanism of suppression of melatonin production and activation of attention by light is particularly important. The pupils are wide open in the dark and the efficiency of light interception is therefore high. No consensus has yet been reached on the safe intensity of disturbing light ('circadian safety'). It is likely to be in the fractions of lux and will depend on the spectral composition, the time course and the spatial distribution of the disturbing light stimulus. It is reported that exposure to intense light during the day improves the body's resistance to disturbing light at night. In health and social care settings, light at night is essential, both for care and safety reasons. Lack of light during the day and disturbing light at night contribute to disruption of the human circadian rhythm.

CHRONOBIOLOGICAL LIGHTING (CHBO)

The previous paragraphs show that daylight and artificial light in buildings is usually not intense enough for the effects of light on circadian rhythms. Artificial lighting designed with extra-visual effects in mind must be able to provide an intensity of 1000-2500 lx during the day and as low as 'legal' at night (e.g. 50 or even just 5 lx). At high intensities, light of a cool tone is natural, whereas at low light levels, warm tones are pleasant. Such luminaires contain several types of light sources and, ideally, a control unit with a built-in clock that automatically controls the intensity and tone of the light during the day according to an embedded algorithm. So when the switch is flipped, the luminaires "know" what time it is and how they should be lit at that time of day. Other names are biodynamic, algorithmic or biologically efficient(7) lighting. This is essentially an indoor replication of the outdoor lighting conditions of a particular time of year on the scale required. Such lighting synchronises the circadian rhythm, complements the low light stimulus in winter, does not hinder the onset of melatonin in the evening and thus contributes to the well-being of its users. It is a modification within the framework of building environment technology, but not a treatment. It can be used from the home to healthcare and social services. However, improving concentration during night shifts needs to be balanced with good sleep hygiene.

CHRONOBIOLOGICAL PHOTOTHERAPY

(CHBFT) IN PSYCHIATRY

Phototherapy as a method of chronobiological treatment is the properly timed application of light of appropriate intensity and spectral composition to a person for the purpose of treating depression or adjusting the sleep/wake rhythm. It finds use in the treatment of depression, not only seasonal but also non-seasonal depressive disorder and bipolar disorder. It has a more rapid onset of action (days) than psychopharmaceuticals (weeks), but its effect wears off after a few days if discontinued, so regular administration is necessary. Rapid improvement is subsequently stabilised by medication. It is a relatively successful method: 50-66% of patients with various types of depression respond to it. It has a rapid onset, typically on the first day. Within 5-7 days, it is obvious if the patient is not responding to therapy. In seasonal affective disorder (SAD), chronobiological phototherapy has comparable results to SSRI-type antidepressants. Another name is bright light therapy (BLT).

Indications: seasonal depression, non-seasonal depression, depressive phase of bipolar disorder. As adjuvant therapy in schizophrenia, schizoaffective disorder and dementia, including Alzheimer's.

Contraindications: retinal dystrophy, macular degeneration.

Relative contraindication: manic phase of bipolar disorder unless the patient is taking a mood stabilizer.

Side effects: conjunctival irritation, nausea, headache.

For details, see Recommended Practices for Psychiatric Care IV from page 191.

Bright light therapy has a tradition in our psychiatric practice since the 1980s. Today, it is used by many psychiatric departments in the Czech Republic and the treatment is covered by public health insurance companies as performance No. 35115, which was introduced by the reimbursement decree 326/2014 Coll.

BRIGHT LIGHT IN PATIENTS WITH DEMENTIA

A study(8) has shown that bright light improves the quality of life of clients in elderly care facilities, reducing the consumption of hypnotics and the cost of night staff. A number of sub-studies have already been conducted on the topic of bright light in clients with Alzheimer's disease. Among the most commonly reported benefits of bright light are:

• adjustment of circadian rhythm,
• reducing agitation,
• reducing confusion,
• reducing the frequency of sundowning syndrome,
• limiting napping during the day,
• improving the quality of sleep,
• improving cognitive function,
• reducing problem behaviour,
• improvement in psychiatric symptoms.

Some studies examine bright light alone, others in combination with, for example, walking, exercise, melatonin, vitamin B12 or other treatments. Most of these studies are in the early and middle stages of the disease. Virtually all studies from the 1990s to the present day indicate that further research is needed. The Cochrane Library meta-study(9) states, "There is insufficient evidence to justify the use of bright light therapy in persons with dementia. Further research should focus on the repeatability of the presenting effect on improving activities of daily living and on determining the biological mechanism by which bright light therapy achieves this improvement." Some more recent studies(10) also point to the fact that there are not yet established standard practices for the use of bright light in patients with dementia. The great advantage of bright light is minimal side effects - only in the form of conjunctival irritation, nausea or headache, and only for a few days while the client adapts to the higher light intensity. If the client does not go out for a long time, the intensity should be increased gradually. Although Alzheimer's disease irreversibly damages the brain, comorbidities (depression, circadian rhythm disturbances, delirium) can be improved in some cases with bright light. Case studies (some unpublished) show how surprising results can be achieved by adding a light stimulus.

The benefit of bright light for a particular client depends on the course of their disease, the existing light conditions, the state of their visual system and their tolerance to bright light. The course of Alzheimer's disease is highly individual, with different brain centres affected to different degrees, rates and sequences in different patients. Statistically, it is difficult to predict whether a particular client will benefit from bright light. With minimal side effects and respecting contraindications (see ChBFT in psychiatry), the benefit for a particular patient can be verified experimentally - with only the risk of possible short-term discomfort.

Bright Light currently offers several progressive facilities for clients with dementia in the Czech Republic, most often in the form of chronobiological lighting of common areas or client rooms.

TECHNICAL EQUIPMENT

The most widely used are portable table and mobile bedside fixtures. The dimensions of the illuminating surface are usually about 30 × 60 cm and the power input up to 100 W. Manufacturers usually indicate the distance from the luminaire at which the illuminance reaches 10 000 lx, which is usually a few tens of centimetres and corresponds to an application time of 30 minutes. The advantage of these products is their mobility, which allows them to be used successively by several users. However, they restrict the user's movement to a large extent during the application period.

Ceiling luminaires with bright light for permanent installation are used both in client rooms and in public areas where they create a distinctive lighting environment. Intensities tend to be in the order of several thousand lux at eye level and the expected residence/operation time is several hours per day.

Other devices include light goggles, caps with a light label, light tables, mobile luminaires above the bed or experimental devices for applying high-intensity light through closed eyelids.

ENERGY INTENSITY

Bright light luminaires provide several times higher intensity than conventional indoor lighting. For portable luminaires, energy costs are on the order of tens of pennies per application. For fixed installations, an input of 0,5 kW per user can be considered when operating for several hours per day. The need for light is greater in the winter months when lighting contributes to space heating. Its use is minimal in the summer, when it would in turn increase cooling consumption.

CONCLUSION

Bright Light has its place in the care of clients with dementia. With minimal side effects, there is nothing to prevent its wider use in improving the environment for clients. Photobiological safety has been part of the pre-market conformity assessment of luminaires for many years. Luminaires for bright light application are available on the market. The much sought after standard procedures for the use of bright light can be brought about by careful processing of experience from practice, for which both potential benefits and minimal adverse effects speak.

Keywords: phototherapy, chronobiology, photosensitive retinal ganglion cells, circadian rhythm, depression, dementia, Alzheimer´s disease

Literature:

1. DIN SPEC 5031-100:2015-08 - Physics of optical radiation and illumination technology: Part 100: Non-visual effects of transocular light on humans - quantities, markers and effective spectra. Berlin: Beuth Verlag 2015.
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6. EN 12464-1:2012. Light and lighting - Lighting in work areas - Part 1: Indoor work areas. Prague: ÚNMZ 2012.
7. DIN SPEC 67600:2013-04. Biologically efficient lighting - design guidelines. Berlin: Beuth Verlag 2013.
8. Sust, et al. Improved quality of life for resident dementia patients: the St. Katharina research project in Vienna. Zumtobel Research 2012.
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Author. Antonín Fuksa, Blue step spol. s r.o.
Published in the journal Geriatrics and Gerontology, 2017; 6 (2)