1. Clocking the body
The internal mechanism controlling our wake and sleep cycle is known as our body clock. More specifically, it is a small tissue located in our hypothalamus called the suprachiasmatic nucleus, or SCN for short. The SCN is our circadian pacemaker. It regulates a vast array of neurological and physiological processes such as melatonin, cortisol, body temperature, heart rate, and metabolism. The harmony of these rhythms is what makes us feel alert, awake and ready for activity during the day, as well as preparing us for sleep and recovery in the evenings.
These processes operate on our circadian rhythm; an endogenoustiming of roughly 24 hours. Because this rhythm is not exactly 24 hours, our bodies use environmental cues, such as light, to synchronise with the Earth's day/night cycle. This is what allows us to acclimatise to new time zones and adapt to changes in seasonal day lengths. Without this ability to synchronise to the light/dark pattern, we would find it extremely difficult to sleep and wake at regular intervals, and would always be fighting the urge to wake earlier or sleep later. This need to synchronise to the light/dark pattern of our environment is not unique to humans; in fact, every living organism uses light to sync their biological metronome. For us, the way in which we receive this information is through the eye.
2. More than vision
The function of the human eye in vision is not a new concept. However, only recently have we understood that our eye is doing much more than allowing us to form images. A newly discovered group of photoreceptors called intrinsically photosensitive retinal ganglion cells (ipRGCs for short) has prompted scientists to rethink the role that light plays in our biology. These photoreceptors are close cousins of the more commonly known rods and cones. They respond to light similarly to rods and cones; however, they are sensitive to very particular bandwidth of light around 480-490nm. Moreover, unlike rods and cones, ipRGCs are non-image-forming and do not contribute to visual acuity. Even though they convert photons into electrical signals (like rods and cones), these signals are sent, via a unique biological pathway, to the SCN rather than the visual cortex.
Research has revealed that stimulating the ipRGC's can actually adjust the timing of our circadian rhythm. Experiments that explored the impact of light between 480-490nm found that melatonin secretion is stalled, thereby delaying the onset of sleep. Even people who are visually blind but with their eyes intact are affected in the same way under these conditions. This understanding of the role light plays in our circadian entrainmentraises a lot of questions around the importance of our daily 'light diet'.
3. Let there be light
Excluding those who spend winters living at extreme latitudes, we can safely say our days are filled with light. Some of it natural but most of it artificial to support the indoor environments we inhabit. Consider daylight for a moment: we know it is spectrally rich in all wavelengths within our visual range. In fact, daylight contains a vast range of radiation well outside our visual range as well, such as infrared and ultraviolet. By contrast, modern artificial light is 'tuned' for visual efficiency, with minimal energy wasted. If we consider LED as our current light source at home and in the office and look at a spectral power distribution curve (SPD), it shows us exactly where this energy is concentrated.
The 4000K LED SPD shows us that there is a large amount of energy located down the shortwave 'blue end' around 450nm, with large amounts of energy as well in the 515nm-590nm range. This is mainly due to the fact that most LED's beginning life as a blue-based diode which then has a phosphor added to it to produce a wider spectrum of colours. This curve also peaks at the same bandwidths that our cones are most sensitive to;
- Short wave cone peak sensitivity (SWC) - 445nm
- Medium wave cone peak sensitivity (LWC) - 510nm
- Long wave cone peak sensitivity (LWC) - 565nm
Putting energy into these bandwidths whilst minimising any energy elsewhere will result in an optimal light source for visual efficiency. This light source is great for vision, however not great for circadian entrainment during the day. We can see that energy at 490nm, the area that our ipRGCs are most sensitive to, is lacking. That is because before understanding the relationship between our ipRGCs and body clock putting light here would be deemed as wasted energy. However, knowing what we do now, we realise that our buildings using current artificial lighting technology are visual efficient but biologically poor.
One solution is to allow more natural light into our buildings to supplement artificial lighting; however, this is not always practical or cost-effective. Daylight comes with a large amount of 'extra' energy in the form of heat and UV, which can be difficult to manage and potentially harmful to the occupants. If correctly managed, sitting by a window would significantly improve the lighting environment for an individual. But what percentage of occupants have access to a window seat?
4. Brighter days, darker nights
Although biologically rich light during the day is required to synchronise our body clock, to much night at night can have the opposite effect, desynchronising our circadian rhythm. In nature, once the sun sets, we are plunged into darkness. However, we have now introduced the concept of 'light after dark' into our homes and buildings. This means our body clock is receiving conflicting signals. Our homeostatic system has been building up sleep pressure all day and our body is preparing for rest and repair. However, our ipRGCs are telling our SCN that it is still light out there and to delay the onset of melatonin. This can cause delayed sleep phase disorder (DSPS), which in turn can heighten the risk of a vast range of illnesses.
Traditionally light after dark was not a concern. Light sources such as candles, oil lamps, and even incandescent light bulbs did not give off much light and contained very little light at 490nm. However today the combination of bright, cool colour LED lamps and screen-based media devices mean our evenings are full of daytime signalling blue light. This is reflected by a decline in sleep quality globally. According to a 2019 global survey on the sleep of 13,000 adults across 13 countries, 80% wanted to improve their sleep quality, 67% reported waking up at least once during the night, and 63% reported they sleep longer on the weekend to catch up on missed sleep during the week. The average sleep time across all 13,000 participants was 6.8 hours on weekdays and 7.8 hours on weekends, both short of the 8 hours recommended by the World Health Organisation. This is an indication of a rising trend in sleeplessness which has been linked to a range of medical ailments such as depression, anxiety, obesity, heart disease, cognitive illness and even certain forms of cancer.
There are many things we can do to improve sleep, but one thing that will significantly improve the timing and quality of our sleep is brighter days and darker nights - Plenty of natural or biologically rich artificial light during the day, and minimal levels of daytime alerting blue light at night. This will provide our body with the right light, at the right place, and at the right time to ensure we are properly in sync with our environment.