**12. Occupational stress-related sleep disorders IN frontline health workers during COVID-19 pandemic: roles of prolonged exposure to artificial light**

Work at night is practically impossible without light exposure. Sequel to the advent of light in 1860, exposure to anthropogenic light has been one of the major challenges in the modern world. Besides being an electromagnetic wave, ambient light is one of the most potent synchronizers of internal rhythms [120]. Shift in light/dark cycle by 6 hours orchestrated desynchronization that spanned for more than 6 days in rodents [116]. This indicates the tendency of light to shift circadian phase. In other studies, exposure to irregular lighting schedules have been claimed to cause prolongation of estrous cycle, lengthening of follicular phase, and an increase in estrous cycle ratio [121–124].

Furthermore, exposure to light is one of the underlying mechanisms of shift work induced sleep problems. Exposure to anthropogenic nocturnal light causes repression of Arylalkylamine N-acetyltransferase (ANAT), an enzyme that is responsible for melatonin secretion by the pineal gland. The absence of melatonin inhibits normal nocturnal sleep. Studies in both human and animal studies have extensively documented the adversities associated with light-induced suppression of melatonin [124]. Melatonin is known to interact with its MTI and MT2 receptors resulting in a decrease in cyclic adenosine monophosphate (CAMP). MTI is found in hypothalamic nuclei where it decreases neural discharge. Melatonin reduces the firing of ascending reticular fibers which project via hypothalamus to the cerebral cortex and thus inhibit consciousness and alertness while promoting sleep [123, 124]. This results in disruption of normal circadian sleep (**Figure 2**).

The hormonal consequences of light-induced melatonin suppression have been reported. Davis et al. [111] showed that health workers on night shift duty exhibit low levels of urinary sulfatoxymelatonin and since melatonin exerts regulatory influence on gonadotropins. The authors also found high urinary levels of FSH and LH in the same people. Increased prolactin level was recorded in women exposed to lengthened lighting period [125, 126]. In rat studies, exposure to constant light may modulate suprachiasmatic *PER2* expression. *PER2* is a member of the PERIOD protein. PERIOD is a circadian protein that combines with cryptochrome (*CRP)* to form a dimer which then acts in a negative loop to inhibit brain and muscle arnt like protein (BMAL) and circadian locomotor oscillator cycles of kaput (CLOCK), which positively promotes the production of PERIOD and Cryptochrome [115]. Blue light has been reported to exert the greatest suppressive effects on melatonin because blue light provides sufficient stimulus for the suppression of ANAT [117]. Light-related suppression in melatonin is due to a reduction in postganglionic noradrenergic neural discharge to the pineal gland.
