For millions of years prior to the widespread use of electricity, humans were exposed to minimal amounts of nocturnal light in comparison to modern times. During clear starry nights, light exposure was limited to around 0.001 lux, and no artificial light sources existed until the use of fire, an ability that is estimated to have been acquired by Homo sapiens approximately 250,000 years ago (Table 1) (Bedrosian & Nelson, 2017; Sevens et al., 2007). As a result, populations across the world were fully entrained to predictable daily cycles of darkness and light, depending on season and latitude (Stevens et al., 2007). In turn, these light cycles allowed for the rhythmic synchronization of various physiological processes to the external environment, a phenomenon that is integral for the maintenance of health and wellbeing. However, since the invention of the electric light bulb at the dawn of the 19th century, we have witnessed an unprecedented light revolution through which nocturnal light exposure in industrialized societies has spiked, and the true physiological discrepancy between day and night has incrementally begun to blur (Bedrosian & Nelson, 2017; Kyba et al., 2017). Consequently, our internal biological clock, known as the circadian rhythm, has desynchronized itself from the natural external light environment, and it is becoming clear that this can exert significant negative effects on mood and mental health (Bedrosian & Nelson, 2017; Fonken & Nelson, 2014).
Table 1 – Intensity of nocturnal light sources
|Source of nocturnal light||Intensity (lux)|
|Clear starry night
Residential side street
Main road lighting
9.7-inch tablet computer
24-inch LED computer screen
Average living room
(adapted from Bedrosian and Nelson, 2017)
Circadian regulation and nocturnal light exposure
Research has demonstrated that a large range of physiological functions are influenced by an internal biological clock known as the circadian rhythm, stemming from the Latin words circa meaning “close to”, and diēm meaning “day”. This pacemaker acts like the conductor of an orchestra, instructing and scheduling cells to complete specific tasks at specific times. Since the inception of the field of chronobiology (i.e. the study of biological rhythm and time), it has been discovered that the circadian rhythm is regulated by specialized, light-sensitive cells that are found in the eyes. These cells are called intrinsically photosensitive retinal ganglion cells (ipRGCs), and in response to light, they activate a region of the brain called the suprachiasmatic nucleus, often referred to as the central master clock. Here, there are around 50,000 neurons that recruit proteins to keep time like a Swiss watch, but unlike most watches, this one is responsible for the temporal control of processes including core temperature, blood pressure, metabolism, sleep-wake cycles, and mood (Bedrosian & Nelson, 2017; Fonken & Nelson, 2014).
The circadian rhythm is in fact influenced by a range of external environmental cues called Zeitgebers, meaning “time givers” in German. These cues include exercise, feeding patterns, social cues, and as was mentioned, light exposure (Blume et al., 2019; Toh, 2008). In fact, light exposure is the strongest external input influencing the synchronization of the circadian rhythm, and it can be influenced by light far less intense than sunlight (Fonken & Nelson, 2014). Importantly, ipRGCs are most sensitive to light with a wavelength of around 480nm (i.e. what we see as blue light), which facilitates the ability to physiologically differentiate between day and night given the colour spectrum of sunlight. Once the sun begins to set, blue light scatters in the atmosphere, and only longer-wavelength light readily reaches the surface (Bedrosian & Nelson, 2017). This drop in light exposure, particularly blue light, signals the physiological transition into night-time, at which point melatonin blood concentrations rise, body temperature falls, sleepiness increases, and appetite decreases (Kraus, 2016). However, in this industrialized age that depends on electrical illumination for work and domestic life, nocturnal light exposure has shifted the timing of the circadian pacemaker such that it is no longer synchronized to natural cycles of light (Bedrosian & Nelson, 2017; Stevens & Zhu, 2015).
In present times, devices including computer screens, smartphones and televisions dominate environments both at home and in public. Studies also report that during the second half of the 20th century, the use of outdoor lighting grew up to 6% every year, culminating in a situation where 99% of those living in the United States or Europe are exposed to light pollution (Bedrosian & Nelson, 2017; Kyba et al., 2017). In addition to this, a particular concern is the recent drive to switch out incandescent bulbs for light-emitting diodes (LEDs) with the intention of reducing energy and maintenance costs. Relative to incandescent bulbs, many LEDs produce blue-shifted light spectra to which the circadian system is particularly sensitive. In fact, 29% of 4000K LED lighting is emitted as blue light, and these lamps are at least 5 times more powerful in influencing circadian rhythm than high pressure sodium lights. In the US alone, approximately 10% of all existing street lighting has already been converted to LEDs, and it is intended that the rate of conversion will continue to increase. Similar transitions are also taking place around the world, for example in Milan, which is readily observable when comparing images taken from the International Space Station in 2012 and 2015 (Figure 1) (Kraus, 2016; Stevens & Zhu, 2015).
Figure 1 – Images of transition to LEDs in Milan, Italy (NASA/ESA)
Finally, additional sources of nocturnal light exposure include engagement in nightshift work, as well as the use of smartphones and other visual display units. Among developed societies, 15-20% of the adult population works nightshifts, during which they are exposed to nocturnal light in a chronic or rotating pattern. Regarding the use of illuminated screens, the National Sleep Foundation reports that 34% of children and 36% of parents leave electronic devices on in the room while sleeping (e.g. computers and televisions), and another survey found that 90% of American adults use electronic devices within an hour before sleep at least a few times per week (Bedrosian & Nelson, 2017; Chang et al., 2015).
Circadian desynchronization and mood disorders
While the underlying mechanisms by which circadian rhythmicity is disrupted are not fully understood, growing evidence is supportive of the notion that desynchronization through nocturnal light exposure might play a role in the development of mood disorders, which are on the rise in industrialized societies around the world (Stevens & Zhu, 2015). In fact, mistimed light exposure and disrupted circadian rhythmicity have long been associated with mood disorders, particularly depressive disorders and bipolar disorder (Bedrosian & Nelson, 2017; Lyall et al., 2017).
Multiple lines of reasoning corroborate a relationship between circadian desynchronization and mood disorders. Importantly, the circadian rhythm regulates a range of systems that influence mood and are altered in mood disorders, including the limbic system, the hypothalamic-pituitary-adrenal (HPA) axis, and monoamine neurotransmitters (Bedrosian & Nelson, 2017). Monoamine signalling (e.g. with serotonin) is a major anti-depressant drug target since this function is believed to be impaired in patients with major depression, and these neurotransmitters display patterns of circadian regulation in terms of concentration, release, and expression. Given these findings, impaired monoamine transmission may represent a potential mechanism linking depressive moods and nocturnal light exposure. Moreover, studies report that light-induced circadian desynchronization can elicit negative effects on sleep, and sleep disruptions are a diagnostic criterion for major depression, bipolar disorder, post-traumatic stress disorder, and generalized anxiety (Bedrosian & Nelson, 2017; Blume et al., 2019). Interestingly, it has also been found that patients admitted for psychiatric emergencies are more likely to display depressive symptoms if they recently travelled across time-zones (Bedrosian & Nelson, 2017). Finally, studies have shown that even minimal exposure to nocturnal light at home (5 lux) is significantly associated with symptoms of depression, and similar findings are reported in studies using hamsters (Obayashi et al., 2017).
Regarding the effects of nightshift work, epidemiological studies consistently link this to symptoms of depression, and this is significantly correlated to light exposure (Bedrosian & Nelson, 2017; Obayashi et al., 2017). In fact, workers undergoing shift work are significantly more likely to suffer from depressive episodes, and working nightshifts for more than 20 years results in an increased lifetime risk of developing major depression. Additionally, a study reported that a single nightshift elicited mood changes among student nurses (Bedrosian & Nelson, 2017).
Moreover, light exposure from LEDs is reported to interfere with sleep and sleep-related behaviours. For instance, a study found that using e-readers for four hours before sleeping reduces evening sleepiness and morning alertness, increases sleep onset latency, and delays the timing of the circadian rhythm. An EEG experiment also found that exposure to short-wavelength light (e.g. blue light) before sleeping results in shallow sleep, and several other studies report that using a smartphone before sleeping may be associated with decreased sleep efficiency and quality, and longer sleep onset latencies (Blume et al., 2019). As previously discussed, sleep disturbances represent a consistent circadian-related event that is a hallmark of mood disorders, and relatively moderate alterations in sleep-wake cycles can elicit significant changes in mood (Blume et al., 2019; Vadnie & McClung, 2017).
Overall, the invention of electric light was a turning point in human history, allowing for safer, wealthier, and more productive societies. However, the field of chronobiology has consistently lagged behind the world-wide adoption of electric lighting, and now scientists are beginning to unravel its unintended consequences on mental health (Bedrosian & Nelson, 2017). Fortunately, limiting exposures to nocturnal light is achievable, and low-cost solutions exist. These include the use of blackout curtains and sleep masks, removing electronic devices from bedrooms, and adhering to consistent sleep patterns. Additional solutions for nightshift workers include preventing blue-light exposure through the use of specialized goggles or reversing their biological clock by exposing them to bright light-pulses during the night and using dark goggles in the morning to promote daytime sleep (Fonken & Nelson, 2014). Finally, in light of accelerated efforts to transition into the use of LED lighting, the American Medical Association released a report encouraging the minimization of blue-rich environmental lighting due to its adverse effects on human health (Krauz, 2016).
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