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Melatonin is a hormone which is primarily synthesized in the pineal gland and then it goes into
the blood. Its 24-hour rhythm is directly driven by the circadian clock from the SCN to the pineal
gland. So pineal melatonin is synthesized during the night (normal peak 1-3 a.m.), and during the
day, production virtually stops.
Melatonin has been shown in studies in vitro to have antioxidant properties, including
scavenging free radicals, preventing tumour cell growth, and enhancing the immune
response (Brzezinski 1997, Korkmaz 2009, Reiter 2010).
The response of the circadian system to light is dependent on the timing and quality of light
exposure (Rimmer 2000, Thapan 2001, Lockley 2003, Gronfier 2004, 2007, Ozkan 2012, Rüger
2013).
Ill-timed light exposure may result in disruption of circadian rhythms dependent on the type of
light (Stevens 2011). Ill-timed exposures to even low levels of light in household settings may be
sufficient for circadian disruptions. Bedrosian & Nelson (2013) in a review on the effects of light
on mood, concluded that night time exposure to light disrupts circadian organisation and
contributes to depressed mood. Exposure to light at night in home settings is significantly
associated with increased night-time blood pressure (BP) in elderly individuals independently of
overnight urinary melatonin excretion. An increase in night-time BP is associated with an
increase in total mortality, which corresponds to approximately 10 000 annual excess deaths in
Japanese elderly population (Obayashi 2014).
A comparison between the effects of living room light and dim light before bedtime showed that
exposure to ordinary levels of room light suppressed melatonin levels and shortened the duration
of melatonin production in healthy volunteers (18-30 years) (Gooley 2011). Wada (2013) found
that low levels of evening lighting improved melatonin secretion at night, which induced easy
onset of sleep and better quality of sleep in students, including athletes.
It has been suggested that circadian disruptions play an important role in the development of
chronic diseases and conditions such as cancer (breast, prostate, endometrial, ovarian, colo-rectal,
skin and melanomas and non-Hodgkin’s lymphomas), cardiovascular diseases, reproduction,
endometriosis, gastrointestinal and digestive problems, premature ageing, diabetes, obesity,
depression (especially during winter months when daylight exposure is often reduced for those
primarily working in artificial light), sleep deprivation, and cognitive impairment (Haus and
Smolensky 2006, Stevens 2007, Takahashi 2008, Frost 2009, Bass and Takahashi 2010, Boyce and
Barriball 2010, IARC 2010, Kvaskoff and Weinstein 2010, Mahoney 2010, Rana and Mahmood
2010, Poole 2011, Ortiz-Tudela 2012).
When cryptochrome in the retina is exposed to blue light, it undergoes a series of complicated
chemical reactions. One of these intermediates has magnetic properties. It could be a link between
the magnetic stage of cryptochrome in the retina and magnetite in the brain. A disturbance in this
system could be involved in the development of frontotemporal dementia and other mental
disturbances like Alzheimer's disease. There could also be a link between circadian rhythms and
memory dysfunction connected to schizophrenia, type 2 diabetes, and blue light (Størmer 2015).
Breast cancer risk has been consistently associated with various aspects of circadian disruptions
including being exposed to high ambient light during the night (Stevens 2009). Three studies link
breast cancer risk to exposure to non-occupational light-at-night (LAN) in the home (Davis 2001,
Kloog 2011, O'Leary 2006), and significant associations were found for women who did not sleep
during the period of the night where melatonin levels are normally peaking (Davis 2001), or who
frequently turned on the light during the night. An increased breast cancer risk was also found
with increasing bedroom light levels (Kloog 2010).
Dauchy (2011) found that even very low levels of LAN in their animal laboratory disrupted
circadian rhythms and stimulated cancer growth. Reducing LAN restored these aspects of
metabolism.