Light and Health

Light plays an important role in our overall health. Around the year 2000 scientists discovered a new class of photoreceptors in the human eye which had nothing to do with vision. The chronobiologist and circadian neuroscientist had been looking for the environmental timing mechanism which would be able to entrain or synchronize our inner master clock. These newly discovered light sensitive photoreceptors are dedicated to the reading of the environmental light condition and sending a direct signal to the brain for the synchronization of our body’s daily rhythms with the solar light/dark cycle. The process is called photoentrainment.

Within our bodies, billions of biological clocks regulate almost every aspect of our physiology and behaviour, fine-tuning them to various demands of activity and rest. These biological clocks and corresponding processes must all be entrained or synchronized to produce an orchestrated effect or homeostasis. There is a complex synchronizing mechanism at work within our bodies with a rhythmicity close to 24 hours which is called the circadian rhythms. Circadian rhythms include our physiological responses to environmental stimuli, from timing of hormonal release to preparing muscle tissues for maximum physical exertion, from sleep to feeling of hunger, from cognitive performance and alertness to mood fluctuations and everything in between.

The main physiological pacemaker for the entrainment or synchronization of our biological clocks is a master clock located in the hypothalamus region of the brain. This master clock is consisting of two pea size bundles of about 50000 neurons. Together they are called suprachiasmatic nucleus. This master clock is an inner or endogenous mechanism which controls our biological rhythms with an average rhythmicity of 24.2 hours. Meaning most of us have master clocks that have a built-in delay which must be reset every day to synchronize with the natural environment. In a constant light condition i.e., light, or dark, this endogenous pacemaker is free running. Human circadian rhythms have on average a delay of an hours every five days. Consequently, if you lived in a cave, in the dark, you would tend to sleep later and wake up later each day until your day and night switch places. In isolation, this causes no physiological problem but relevant to our environmental living conditions this can cause disruption in sleep and circadian rhythms in relation to our social and work schedules. Therefore, to be in synch with our living environment our master clock must be reset by an environmental or exogenous pacemaker. Light plays the main role for this synchronization or photoentrainment. There are other environmental pacemakers, but we will be considering the most potent and important one which is the light/ dark cycle.

In the morning when the bright daylight hits the retina on the back of our eyes, it activates all our three classes of photoreceptors. The rods and the cones are dedicated to phototransduction for formation of vision, color, motion, and contrast but those newly discovered photoreceptors called the intrinsically photosensitive retinal ganglion cells (ipRGCs) are particularly sensitive to the absorption of short-wavelength (blue) visible light. These photoreceptors in turn, through phototransduction, communicate the blue light information directly to the suprachiasmatic nucleus as the first signal for daybreak. It is then that the master clock suppresses the secretion of melatonin and increases the expression of cortisol to activate the body for the day activity. It is at this point that all other light related day rhythms start. As diurnal species we wake up with sun and are active with light and then we sleep in the dark. When Sun sets, the absence of blue light signal from the retina to the master clock signals the start of night mode. Cortisol secretion subsides while melatonin onsets. With melatonin expression come the rest of the night rhythms. Our bodies have evolved to anticipate the nightfall and daybreak through the circadian system. Untimely light signals can disrupt circadian rhythms and sleep.

Disruption of sleep and circadian rhythms is associated with many life-threatening chronic diseases, neurodegenerative diseases, numerous accidental deaths, gastrointestinal diseases, obesity, cardiovascular diseases, some cancers, and a whole host of mental and behavioural disorders. Published research indicates that seven of top ten causes of death in the developed world are chronic diseases linked to circadian rhythms and sleep disruption. ?

For a robust photoentrainment of the circadian system there needs to be a clear and drastic contrast between the light/ dark periods in each cycle. The daylight, in natural environment, on a clear day in summer in Norway can reach an intensity of 100000 lux while at night it reduces to zero. This blue-rich bright light during the early hours of the day has an advancing or corrective/ positive impact on the circadian rhythms. The robust photoentrainment of the circadian system is dependent on the intensity, spectral power distribution, timing, and duration of the environmental light stimuli. The contrast between very bright light of the day and total darkness at night is of equal importance to photoentrainment.

Major problem is that we do not live in an unmediated natural environment anymore. Today, most of us spend more than 90% of our lives in and about the built environment with profound impact on our health and wellbeing. Comparatively, even in the late 1800s most people spent about 80% of their lives outdoors. The built environment has become our de facto natural environment and its lighting conditions have major disruptive impact on our health. In relation to photoentrainment, most interior spaces are dimly lit during the day and too bright at night. This light environment also lacks the necessary contrast between bright daylight and total darkness of the natural solar cycles. This light environment has a disruptive effect on our circadian rhythms and sleep which have adverse effect on our mental and physiological health. Our brain does not distinguish between daylight and electric light, only the intensity, coloration, timing, and duration of light exposure mater.

?Since the advent of electrical grid and the electric light bulb about 140 years ago, our living environment has changed drastically. We literally extended our days by lighting our nights. As a result, our professional and cultural lives have changed irreversibly. We now could work and socialize way into the early hours of next morning. We now can claim that we are a 24/7 kind of society that never sleeps. Sleep started gaining a negative cultural charge like weakness and laziness. As a result, there is an overt cultural disdain for sleep as sign of social and professional disengagement. ?More than 32% of working adults report sleeping less than six hours while sleep duration recommendations remain at 7 to 9 hours for healthy adults.

Sleep is perhaps the most apparent of the circadian rhythms. Lack of timely and sufficient sleep is an indicator of circadian desynchrony. As discussed above, the disruption of sleep and circadian rhythms are associated with many adverse health effects. As the main environmental entraining signal, light plays a pivotal role in our overall mental and physiological health. Because we live more than 90% of our lives within the built environment, it is therefore, more important than ever to address daylight, light and lighting in the design and engineering process of the built environment to improve quality of life and health for the occupants. Maximizing the advantageous daylight and supplementing it with tuneable lighting during the early hours of the day and minimizing light exposure at night can be a starting point for a human centric design process. Of course, there are economic issues that have to enter the process as well, but the balance needs to tilt towards the health and wellbeing of the occupants. The closer we can mimic the natural light environment within the built environment the better would be the results.

In future posts we can discuss specific strategies and action items that can help us, individually, in improving our circadian health and light intake habits to reduce physiological and psychological stress. ?As well, we will be discussing design and engineering strategies to produce human centric built environments better suited for productivity, focus, rest, healing, health, and wellbeing.?