Why light exposure is so important for mental health (independent of sleep)

“Avoiding light at night and seeking light during the day may be a simple and effective, non-pharmacological means of broadly improving mental health.”

This is the conclusion to a paper published in October 2023 by a group of researchers from universities in Australia, the US, and the UK. It was the largest study ever done (over 85,000 subjects in the UK) on the links between light exposure and mental health.

We should take note.

The study showed that:

Greater light exposure during the night was associated with:

  1. Increased risk for major depressive disorder, generalised anxiety disorder, PTSD, psychosis, bipolar disorder, and self-harm behaviour; and
  2. More severe symptoms of depression, anxiety, and PTSD

Greater light exposure during the day was associated (independent of night-time light exposure) with:

  1. Reduced risk for major depressive disorder, PTSD, psychosis, and self-harm behaviour; and
  2. Less severe symptoms of depression, anxiety, and PTSD

These associations were independent of age, physical activity, cardiometabolic health, sociodemographics, and sleep.

Compared to the light-dark cycles that we evolved in, we now live in a world that’s too dim during the day (we don’t go outside enough) and too bright at night (we surround ourselves with artificial light). This dysregulates our circadian rhythm, which negatively impacts our physiology and our psychology.

To address this deviation from the light-dark cycle and properly regulate your circadian rhythm for better mental (and physical) heath, try to do one or more of the following:

  • Maximise outdoor light during the day (go outside more, especially in the early hours of daylight)
  • Minimise artificial light in the evening (dim the overhead lights, use lamps, turn off the screens)
  • Eliminate light during the night (make your bedroom as dark as possible, avoid switching on bright lights if you have to get up)

To explain the details, let’s take depression as an example. There were two depression outcomes: 1) incidence of major depressive disorder (MDD); and 2) self-reported depression scores (PHQ-9 Score).

Greater light exposure during the night (segment A) was associated with increased risk of major depressive disorder (MDD) in a dose-dependant manner. In other words: more light at night, more depression. People in the fourth quartile (the 25% with the greatest night-time light exposure) were 30% more likely to have MDD than people in the first quartile (the 25% with the lowest night-time light exposure).

Greater light exposure during the day (segment B) was associated with reduced risk of MDD (dose-dependant again). People in the fourth quartile were 20% less likely to have MDD than people in the first quartile.

These associations were independent of each other (meaning that even in those people with the greatest night-time light exposure, greater daytime light exposure was associated with reduced MDD risk) and additive (meaning that those people with the most daytime light exposure and the least night-time light exposure had the lowest risk of MDD).

Greater night-time light (segment A) was associated with higher depression scores and greater daytime light (segment B) was associated with lower depression scores. Again, these associations were dose-dependent and independent of each other.

Now, we do need to be a little careful with this type of observational (cross-sectional) study. It can’t tell us whether or not the light exposure is causing the mental health outcomes. However, the researchers adjusted the data for age, sex, ethnicity, and differing daylight hours across seasons (Model 2), additionally for employment and physical activity (Model 3), and additionally for sleep. None of these adjustments changed the findings. Together with the dose-dependence, this makes causation more likely.

There is, of course, the possibility of reverse causation (mental health problems causing changes in behaviour that result in less light during the day and more light at night), but this is less likely given what we know about the causal mechanisms linking light exposure to circadian rhythms, and circadian rhythms to mental health.

In conclusion, I’ll quote the paper directly:

“Taken together, our findings are consistent with bright daytime light and low night-time light strengthening circadian rhythms as an antecedent to more robust mental health. Patterns of bright daytime light and low night-time light serve to enhance the amplitude and stability of the circadian clock as well as align its timing appropriately with daily activities. As modern humans spend ~90% of the day indoors, our light-exposure patterns are typically less bright in the day and more bright at night than naturalistic patterns across our evolutionary history. Addressing this deviation from our natural light/dark cycles may improve the general mental health of people in industrialized societies.”

Share post

Discover BioMe

Sign up today

Get a weekly science-proven and actionable tip sent straight to your inbox

Related posts