The physiology of all living species is entrained or synchronised to the 24-hour solar day, which consists of a single light-dark cycle. To maintain this periodicity, our internal circadian clock aligns with environmental cues, or 'zeitgebers' (time-givers, German), which may be temperature, physical activities, pathogens, or even alarm sounds. The most powerful zeitgeber for circadian entrainment is light. As a result, organisms exhibit distinct diurnal (daytime), nocturnal (night-time), and crepuscular (twilight period) behaviours.
In sync with the environment
Synchronisation with the 24-hour geophysical day helps in the maintenance of an appropriate phase relationship between environmental time and the circadian system (circa, approximately; dies, day; Latin). This allows diurnal species, including humans, to maintain wakefulness during the day and consolidated sleep at night. At the start of a new day, morning light increases our alertness, whereas twilight and subsequent darkness promote sleep.
Our circadian rhythms become desynchronized with our sleep-wake cycle when we are exposed to light at inappropriate times. Prolonged exposure to light, particularly after twilight, delays the onset of sleep. This results in compromised cognitive functions, impaired social relationships, and an increased susceptibility to infection and disease. To avoid circadian misalignment, we require adequate doses of daylight during the day, and darkness at night.
Daylight is the visible spectrum of solar radiation, ranging from 380 nm to 780 nm. Daylighting is the controlled admission of sunlight into the built environment, which can be direct sunlight, diffused skylight, or light reflected from surrounding elements.
Daylighting is an essential design element that determines both the form and function of a built structure, particularly fenestrations. The design and placement of doors, windows, skylights, and shading devices, the reflectance of room surfaces, as well as occupant behaviour, determine how much light enters a building, its timing, direction, and duration, and whether the daylight is diffused or direct.
Natural light is full-spectrum and dynamic. Its intensity and colour mix change from dawn to dusk, shifting from bright with a strong blue content to awaken us, to soft with a high red content to soothe us. Short wavelengths, which are seen as blue, are the most powerful synchronising agents for the circadian clock. The overwhelming majority of artificial light sources are designed to mimic either daylight or evening light. Their colours remain unchanged due to their static spectrum. The difference in spectral content between natural and artificial lighting is extremely significant with respect to our circadian cycle.
Daylight is also the most sustainable source of light for circadian entrainment. It not only enhances occupant comfort and promotes health and well-being, but it also decreases dependency on artificial lighting by naturally illuminating spaces, reducing energy expenditures. Understanding how daylight regulates the circadian cycle can help inform lighting design in the built environment.
Measuring time at the sub-cellular level
The biological master clock, our internal Circadian Timing System (CTS), which generates and maintains all known circadian rhythms in vertebrates, is housed in the suprachiasmatic nuclei (SCN) of the hypothalamus region in the brain. The SCN generates circadian rhythms at rest and during activity and regulates many behavioural and physiological processes as well. Approximately 20,000 SCN neurons, each with a 24-hour rhythm of activity, synchronise to serve as the body's powerful daily timer.
To optimise cell function and reduce the occurrence of pathologies, the SCN uses photoperiodic information to measure both daily and seasonal time. Although light detection by the retina is required to generate the circadian rhythm, the retinal photoreceptors that lead to visual image formation are not involved. It is the intrinsically photosensitive retinal ganglion cells (ipRGCs) within the mammalian eye that are responsible for SCN photic entrainment.
A match made within
The individual circadian periods vary between 23.5 and 24.5 hours, or a little longer, while the time taken for the Earth to complete one rotation around its axis, measured from noon to noon, is 24 hours. Chronodisruption, or a mismatch between our biological rhythm and the 24-hour solar day, would cause our physiology to go out of sync with our environment.
The circadian rhythm phase-shifts when the body is exposed to mistimed zeitgebers. The phase of the circadian clock at which the stimuli occur determines the amount and direction of the shifts (advance or delay). A phase-delay shift (late bedtime and wake-up time) occurs when the light stimulus occurs early in the night, whereas a phase-advance shift (early wake-up and bedtime) occurs when the light stimulus occurs late in the night. The length of the phase shift is influenced by the brightness, colour, timing, and duration of light exposure.
The shape of the Phase Response Curve (PRC) of light exposure, which illustrates the relationship between a stimulus (light) and an organism's response, has been observed to be comparable across all living creatures investigated to date. This clearly shows that our ability to align our time with the cues provided by the daily light-dark cycle evolved very early in evolution and is vital to life on Earth.
Daylighting for the indoor species
We erroneously believe that we are immune to the adverse effects of living contrary to our internal temporal biology. The societal pressures of 24-hour cities are disrupting our 24-hour biological rhythm, made possible by affordable artificial lighting. This has resulted in architecture that is designed less for daylighting and more for artificial lighting.
We must design our buildings to allow for appropriate daylighting in order to function properly and continually adapt to the demands brought on by the light-dark cycle. This is particularly significant in a world where the human race is progressively becoming an indoor species throughout its life cycle. To achieve this, the full spectrum of stakeholders must be enlightened on how much our health is impacted by our daily dose of daylight.
(This article is the first of a series on ‘Daylighting’ in the built environment.)
(Ann Rochyne Thomas is a bio-climatic spatial planner and founder of the Centre for Climate Resilience - a sustainability and climate change advisory.)