Multiple factors can contribute to different mental health conditions, but in the largest study of its kind, researchers suggest limited light exposure during the day or high levels of exposure at night may be involved

Limited light exposure during the day or being exposed to high levels of light at night may be linked with certain mental health conditions.

“We propose that day and night light exposure are important environmental factors that contribute to an individual’s risk for certain disorders against the background of their genetics and other environmental factors that interact with one another,” says Angus Burns at Monash University in Melbourne, Australia.

“By no means do we suggest that circadian rhythms or light are the sole determinant of mental state, rather that unhealthy circadian rhythms would increase your vulnerability/risk to poorer psychiatric health,” he says.

In the largest study of its kind to date, Burns and his colleagues analysed 86,000 participants of the UK Biobank study.

The participants wore a step counter with built-in light sensors on their wrist for one week. These sensors didn’t differentiate between natural and artificial light. Daytime was defined as 7.30am to 8.30pm.

Next, the participants were ranked and split into four groups based on their light exposure.

They also self-reported any mental health conditions via a questionnaire that asked whether they had been diagnosed with a particular condition or if they met a list of criteria.

Compared with the participants in the lowest quartile for light exposure at night, those with the highest exposure were 34 per cent more likely to have post-traumatic stress disorder (PTSD). A statistical analysis suggests this wasn’t a chance finding.

PTSD can cause sleeping difficulties, such as insomnia, which could explain the link between light exposure at night and the condition. According to Burns, the team accounted for the participants’ activity levels, measured via the step counter, to somewhat gauge their wakefulness during the night. The link between light exposure at night and PTSD still stood.

Since the research was carried out, the team has done an additional unpublished analysis that adjusts for the effects of sleep duration and quality, says Burns. “When we did this, the relationship between night light and PTSD was also unchanged, remarkably,” he says. “What this means is, irrespective of how much time people sleep or if they wake up during the night, night light exposure predicts greater PTSD risk.”

Read more: UK schools are teaching teenagers about mental health in the wrong way

People with PTSD often have nightmares and may find comfort in sleeping with a light on. According to Burns, this light exposure may have a “direct effect on arousal leading to poorer sleep and a direct effect on circadian rhythms, which contribute to worse psychiatric outcomes”.

Our circadian rhythm, or body clock, regulates “virtually all areas of physiology and behaviour”, the researchers write in their paper.

Helen Burgess, at the University of Michigan, says that increased night-time light exposure may be implicated in PTSD itself or it could be used as a source of comfort that inadvertently affects a person’s circadian rhythm, potentially affecting their condition.

The results also show that the participants with the highest light exposure at night were between 21 and 30 per cent more likely to have major depressive disorder, self-harm behaviours, generalised anxiety disorder or psychotic experiences, compared with the participants with the lowest night-time light exposure.

Those with the highest daytime light exposure were 18 to 31 per cent less likely to have major depressive disorder, self-harm behaviours, PTSD or psychotic experiences, compared with the participants with the lowest daytime light exposure.

A reduced ability to experience pleasure may mean that people with conditions such as depression spend less time outdoors, thereby receiving lower levels of daylight, says Burns.

He believes the researchers took this into account by adjusting for the participants’ physical activity levels. “When we condition on the different levels of activity people engage in, day and night light are still associated with generally greater risk of psychopathology,” says Burns.

The study’s large sample size supports the validity of the results, says Burgess. But all the participants were aged 37 to 73, so the potential effect of light exposure on people outside of this age bracket is therefore less clear, she says.

Burgess also questioned the accuracy of measuring light exposure via a wrist-worn device. She has been involved in trials that used photosensors worn around the neck to gauge the amount of light entering people’s eyes.

Need a listening ear? UK Samaritans: 116123; US National Suicide Prevention Lifeline: 1 800 273 8255; hotlines in other countries

Reference: medRXivDOI: