‘Confused and unwell’: Light exposure’s impact on mental health

New research has revealed that exposure to artificial light during the night that disrupts circadian rhythms can have a significant impact on mental health and psychiatric disorders, including anxiety, bipolar, PTSD severity and self-harm.
 
On the other hand, exposure to light during the day can counteract risk of the same mood disorders and improve wellbeing, presenting one way to improve mental health without the use of medication.
 
Published a day ahead of World Mental Health Day, the research is the largest of its kind to date and states that light is ‘the primary input to the circadian clock, with daytime light strengthening rhythms and night-time light disrupting them’. Hence regular light exposure may be an environmental risk factor for susceptibility to psychiatric disorders.
 
Study author Associate Professor Sean Cain from Monash University’s School of Psychological Sciences and Turner Institute for Brain and Mental Health, said the findings represent a shift in human behaviour and biological systems in the modern world.
 
‘Our brains evolved to work best with bright light in the day and then with almost no light at night,’ he said.
 
‘Humans today challenge this biology, spending around 90% of the day indoors under electric lighting which is too dim during the day and too bright at night, compared to natural light and dark cycles.
 
‘It is confusing our bodies and making us unwell.’
 
For the study, 103,720 UK Biobank participants completed a seven-day assessment measuring activity through light and movement (actimetry) and light monitoring.
 
After those with poor quality or ‘unreliable’ accelerometry, sleep and light data were excluded, the remaining 86,772 participants (57% female) were examined for exposure to light, sleep, physical activity and mental health, with the impact of night light exposure independent of demographic, physical activity, season and employment.
 
A higher exposure to night-time light was associated with increased risk for major depressive disorder, generalised anxiety disorder, PTSD, psychosis, bipolar disorder, and self-harm behaviour, and greater daytime exposure showed reduced risk for the same conditions.
 
Specifically, the risk of depression for high night-time light exposure increased by 30%, as well as poorer self-reported mood and wellbeing.
 
Director of Deakin University’s Institute for Mental and Physical Health and Clinical Translation Professor Michael Berk, who was not involved in the research, told The Age it is well known that disruptions to circadian rhythms cause disruption to mental health measures.
 
‘We know if you want to trigger an episode of bipolar, the very best way to do that is muck with the circadian clock,’ he said.
 
Meanwhile, the UK study found high exposure to light during the daytime was shown to reduce depression risk by 20%, and participants self-reported better mood and wellbeing. Similar result patterns were shown for generalised anxiety disorder, psychosis, bipolar disorder, PTSD and self-harm behaviour.
 
The authors, whose findings are consistent when accounting for shiftwork, sleep, urban versus rural living and cardio-metabolic health, conclude that avoiding light at night and seeking light during the day may be ‘a simple and effective, non-pharmacological means’ of broadly improving mental health.
 
‘Our findings will have a potentially huge societal impact,’ Associate Professor Cain said.
 
‘Once people understand that their light exposure patterns have a powerful influence on their mental health, they can take some simple steps to optimise their wellbeing.
 
‘It’s about getting bright light in the day and darkness at night.’
 
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