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Daylight and nightlight are associated with psychiatric disorders, according to a large naturalistic light study

Tone Elise Henriksen is a Science Advisor of the Good Light Group. She works as a consultant psychiatrist at Valen Hospital and postdoctoral fellow at the Department of Research and Innovation, Fonna Health Trust, Norway.

Right light at the right time is health-promoting and important for all people, but possibly even more so for people with psychiatric disorders. Light during daytime activates the brain and rest of the body at a time we are adapted to be active and awake. To seek light during the day also helps reinforce our circadian rhythms to be synchronised both internally and with the natural 24-hour light/dark cycle of the environment. Light at night promotes activity and wake when the body should rest. Also, light at night has the potential to disrupt the circadian rhythms by sending the wrong daytime signal to the brain’s master-clock. This may happen even if you are sleeping with your eyes shut. The to-date largest naturalistic study on day and night light exposure and the associations with psychiatric disorders, published in Nature Mental Health in 2023, used data from impressive 86 722 participants (from the UK biobank). The paper describes how various levels of light exposure in individuals in free-living conditions are associated with the incidence of a broad span of psychiatric disorders. The authors found that more light exposure during the night was associated with increased self-reporting of the most common and debilitating psychiatric disorders, major depressive disorder, bipolar disorder, post traumatic stress disorder (PTSD), generalized anxiety disorder, psychosis, and self-harm behavior. Higher daytime light-exposure was associated with less reported major depressive disorder, PTSD, psychosis, and self-harm behavior. Moreover, recordings of darker days and brighter nights were associated with poorer mood and well-being.  The strongest associations were found in the brightest night-time light group with a near 30% higher risk of reporting major depressive disorder and self-harm behavior. The authors also found that the day and night light conditions contributed independently and additive to the altered risks of the psychiatric disorder syndromes.

Photo by Jongsun Lee on Unsplash

The one-week recordings were made with a wrist-worn combined activity and light-recorder in the period 2013-2015. It is of note that the study population was on average 62 years. This means that the participants were middle-aged adults before they were exposed to LED-lights and acquired their first smartphone.

After controlling for sleep quality and duration, and several lifestyle and demographic factors, the associations remained significant. Based on the data from this cross-sectional study alone, it is not possible to establish if there is a causal relationship between light exposure and the development of the disorders, whether the light exposure is a consequence of the disorders or whether there is another mediating factor. For example, patients high in anxiety such as PTSD, general anxiety disorder or psychosis may choose to sleep with the lights on, and patients with manic symptoms tend to have lights on when awake at night. However, whether cause or consequence, it is however safe to interpret that many patients reporting psychiatric disorders were exposed to unhealthy light conditions, when they should not. We know that wrong light at wrong time counteracts recovery from psychiatric disorders.  Light exposure interventions are successfully used as treatment for major depression and bipolar disorders. We also know that unhealthy light promotes weight gain, which is a risk factor for the most common cancer types and cardiovascular disorders. These somatic disorders are the main cause of the increased mortality for people with psychiatric disorders, with consistent findings of shockingly 20-25 shorter life expectancy as compared to the general population. Unlike most other known environmental factors affecting health, light conditions can be optimized with low cost and minimal risk. This study is yet another wake-up call that we should talk about light - both in the treatment context of each individual patient and at a societal level.


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