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According to the Black Dog Institute, one in five (20%) Australians aged 16-85 experience a mental illness in any year. The importance of seeking out a healthy (and non-sunburning) level of sunlight in the day is well known, with multiple studies showing those who get a dose of daylight regularly seem to be healthier. But light exposure at night is less understood. It is becoming ever more relevant particularly with a multitude of devices keeping us bathed in light well into the night. In a new study researchers have quantified the effect of day and night light exposure on peoples’ mental health and mood – and the effects are significant.

They used data from the UK Biobank, a group of more than 500,000 people. A subset had a seven-day assessment of their light exposure, and compared that to data on their mental health and general wellbeing. They split participants into quartiles of daytime and nighttime light exposure and were testing two hypotheses – that daytime light exposure is linked to decreased likelihood of mental health disorders and low mood, and that nighttime exposure is linked to increased likelihood.

In the more than 100,000 people analysed, researchers found that their hypotheses were correct. Higher daytime light exposure was connected to lower odds of major depressive disorder and self-harm. Being exposed to more night light was linked to about 30 per cent higher odds of the same, and those with high night light exposure were also more likely to experience anxiety and post-traumatic stress disorder. This held even when researchers adjusted for sex, ethnicity, employment and level of physical activity – and whether or not shift workers were included in the sample. There wasn’t a lot of interaction between day and night light (so even those with the greatest exposure to light in the day had an increased risk of depression if their night-light exposure was high), though they did find that being exposed to light in the day helped to moderate the impact of high night-light exposure on overall wellbeing.

Researchers believe this all ties back to our natural circadian rhythms – and affirms that we should be getting a good dose of bright daytime light while restricting night-time light to support robust mental health. This may also be one of several reasons why other studies looking at physical exercise and time in nature see people get a boost in mood from these activities (often as they come with exposure to natural light) and why high screen time can have a harmful effect on mood.

 

More information

Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 people: Nature Mental Health https://www.nature.com/articles/s44220-023-00135-8

Facts & figures about mental health https://www.blackdoginstitute.org.au/wp-content/uploads/2020/04/1-facts_figures.pdf