15 questions on daylight and body functions

Does artificial light give us the same benefits as natural light in dark winter days?

Artificial light does not give us the same benefits as natural light, because it has another spectral composition and it is of much lower intensity. During the day it is not harmful, but we do not get the same amount of photons as with natural daylight. However, when artificial light is given at the wrong time, for example when we sleep, then it can have negative effects, especially when the blue portion is high as in computer screens.

Is depressive behavior a consequence of variable light intensity and does it go through desynchronization of the Suprachiasmatic nucleus (SCN)?

We did not test light intensity in our experiments with mice, however, in humans the intensity plays a role to a saturation point. Whether the SCN is involved is not entirely clear yet, but it appears that it may not be crucial in mice. Other brain regions that are getting the light signal directly from the eye, and not via the SCN, are probably regulating mood related behavior.

In light therapy, is blue light more effective than getting 10.000 lux in the morning?

Yes, blue light (480 nanometer) is the most effective wavelength since it activates specifically the intrinsic photosensitive retinal ganglion cells in the retina. Also, other wavelengths have a more moderate influence via other photoreceptors such as the rods and cones.

Apart from SCN, which are the other areas of the brain that are affected by daylight?

Other areas that are affected by daylight are the Perihabenula (PHb), Ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL) and from there to the lateral habenula (LHb) and from there to the ventral tegmental area (VTA). Also, there may be even more brain nuclei affected by light we do not know.

How long does a light pulse need to be to synchronize clocks?

In mice, this pulse can be very short, as one minute already has an effect. In our experiments, we do 15 minute light pulses for mice. In humans for treatment of seasonal affective disorder this is around 30 min (10’000 lux), and for 2500 lux, one hour of light is required (Wirz-Justice, A., Benedetti, F., Terman, M., 2013).

Is it possible to study shiftwork in mice?

Yes, Roelof Hut in the Netherlands has set up a food for work paradigm that allows to force mice to be active at irregular times.

How do I know if my biological clock is in or out of rhythm?

One usual indicator of a challenged biological clock is non-restorative sleep. Ones’ sleep can be too short, is terminated to early, interrupted at night, or starts later as intended. There is a steep overlap with the diagnostic criteria for insomnia. Sleep timing especially is a common outcome regarded to detect biological clock related sleep problems, especially meaning when sleep times vary much across days, when sleep timing becomes irregular. The difference between sleep on work-free days and workdays has been coined ‘social jetlag’, which has been associated with adverse health and performance outcomes. Since sleep disturbances are virtually connected to almost all health and performance outcomes, also changes in health status across weeks and changes in performance can be indicative of an underlying disturbance of the biological clock. A common behavioral change after a poor night of sleep indeed is changes in mood (albeit, of course, mood swings can have various origins).

When the circadian clock is disrupted through light in the false time, which hormones are most directly affected?

The usual suspect one looks at is melatonin. Light at the false time is light at times after dusk and before dawn. That time frame is evolutionary conserved for melatonin to signal the length of the night (at the same time coding for the length of the sun-day) to the body (i.e. to all clocks in a body). There is more and more research showing that large parts of our endocrinology are compromised, resulting in hormones that are time shifted in phase and/or altered in amplitude. Both signals (the timing and amplitude) appear crucial to convey environmentally coded circadian timing to the body.

Do you integrate Vitamin D in the picture of rhythms? Can the seek for Vitamin D be illustrated along the circadian rhythms?

There is, of course, a strong link between exposure to daylight and synthesis of Vitamin D in the skin. Vitamin D status might also serve an indicator of daylight exposure, whereas study findings are not that straightforward. As far as I know Vitamin D is not considered as input to the circadian clock machinery.

Why is there a drive for increased eveningness during the puberty?

Hormonal processes are suspected to be involved, but the picture is blurry. Interestingly, the same phenomenon has been observed in non-human primates and rodents.

What are the quintessential biological, social and demographic factors that should be considered for inter-individual comparison, appreciating the heterogeneity between individual trend of chronotypes?

Chronotype varies with age, sex, genetic makeup and place of residence/(day)light exposure. The same holds for circadian phase, e.g. assessed via the dim-light onset of melatonin in the evening prior sleep. There are different metrics to assess circadian phase and chronotype, some are subjective others are objective, each most likely affected by these variables but to different degrees. Most studied in this context is the influence of light, with newer research also showing modifying input from food intake and physical activity. The field distinguishes further here between central (the suprachiasmatic nucleus and connected neurons in the brain) and peripheral clocks (clocks in the body but outside the brain). The quintessential factors are not yet all elucidated. Social factors are all those factors that can indirectly interfere with the process of entrainment, the process of setting the biological clock. The human body functions on the basis of finely synchronized biological processes, which in turn are influenced by Zeitgebers (most dominant is light) from the environment. However, humans often manipulate their environment in terms of time (e.g., shift and night work, clock change to daylight saving time, early school start, etc.), which easily can lead to a desynchronization between social and biological time. The result can be serious and costly consequences for sleep, wellbeing and health.

What is your take on facade design and optimal use of daylight?

Successful facade design, for me, is to find a way to keep a view out and to provide variety of daylight to keep sufficient levels of daylight for visual tasks and good physiological and psychological functioning. How to do it is a complex challenge each time. Excessive glare and thermal discomfort result in end-user’s willingness to cover glazed areas to control direct sunlight. Designers are aware that designing a window is not only to allow daylight inside but also to provide the connection with the outside environment. Windows covered all the time by the users to protect them from unwanted effects of daylight do not fulfill their major tasks. There are some studies based on long-lasting experiments which aim to determine best solutions for color glazing. A comprehensive review of different dynamic window properties can be found in Casini’s publication (2018). Still, there is a long way in front of us to find a way for better daylighting design for the optimal circadian stimulus.

Would windows with wider degree angles be better for daylight entering the building?

The provision of daylight is greater for roof windows but daylighting design is not only about providing a lot of direct daylight into a building. It is about finding a balance and daylight control. Extensive daylight exposure indoors especially in terms of direct sunlight brings issues like glare and thermal discomfort. The optimal position and size of the aperture depends on the building design but also the climate and daylighting availability. In some cases, tilted windows or tilted facade can work beautifully.

What would you suggest in terms of daylight design for nursing homes and eldercare facilities?

My suggestion will be design which includes a lot of greenery (green walls, patios with plans) and apertures with pleasant views and good daylight control. In terms of daylight provision, high contrast scenes should be avoided especially for residents who are visually impaired. Also, a provision of higher illuminance values than those found in recommendations is suggested. Better cognitive performance among elderly people is found under sunny sky conditions. Some studies done by the NTNUs Light and Colour Group suggest that older residents prefer traditional daylight control systems like curtains.

Is the benefit of natural sanitation (from UV) from daylight included in the building design?

There are few studies discussing UV sanitation within the built environment. However, what I can say, is that much of the sunlight spectrum is filtered through window glass. The result is that UV is largely blocked by glass and then absorbed by finishes.