Light and Mental Health

How Light Directly Impacts Mental Health

We have seen how light is important to health through regulating and strengthening our circadian system, better aligning our sleep-wake cycle to the workday, and improving sleep quality. There is a fourth, equally important way that light impacts our wellbeing, though it garners less attention: light directly affects mental health through brain pathways completely separate from the circadian system and sleep.

I was first clued in on this by a Huberman Lab Podcast episode[1] several years ago interviewing Dr. Samer Hattar from the National Institute of Mental Health (NIMH). Hattar’s lab had discovered that light signals, via the ipRG’s, a newly discovered brain center, which he called the Perihabeniular Nucleus (PHb). The PhB then signals various mood centers of the brain. The researchers found that aberrant light exposure altered the brain signaling from the PHb and induced a depressive state.

My curiosity piqued, I tracked down the paper[2]. The experiment was conducted on mice, as nearly all mechanistic studies are, so I wondered if the PHb had been identified in humans. While I didn’t find anything directly answering that, I was surprised to learn that light, still via the ipRGC, signals many different parts of our brain that regulate mood and mental health[3,4]. I won’t pretend to know what each of these brain centers is and their exact function since I’m not a neuroscientist, but what is striking to me is how pervasive and broadly light via the ipRGC signals different parts of the brain. It affects monoamine (e.g. dopamine and serotonin) systems, regions of the brain that affect alertness, and various regions involved in emotion regulation, all through networks completely separate from the SCN.

How light affects mood is curiously similar to light for the circadian system: you need enough light, for the right amount of time, at the right time of day, with the right kind of spectrum[3]. Circadian disruption often accompanies mental health challenges, leading some researchers to wonder if circadian disruption might contribute to the onset or severity of mental health challenges. Conversely, mechanistic studies reveal that dysregulating light in a way that doesn’t disrupt the circadian system can still negatively impact mood[5]. Perhaps, then, aberrant light exposure erodes mental health directly, regardless of circadian disruption’s effects. In practice, it’s likely some combination of both. Practically speaking, it doesn’t matter for us designing and building the built environment. Addressing one (the circadian system) also addresses the other (mental health), and in the real world, the dysregulated light exposure we experience daily disrupts the circadian system and mental health.

Indoor Light Induced Depression?

We have all heard of Seasonal Affective Depression (SAD) and know that the therapy is bright light therapy (BLT). As I continued digging into this topic, I was surprised to learn that BLT is also effective for non-seasonal depression. A 2005 meta-analysis found that bright light therapy is effective for treating major depressive disorder as commonly prescribed SSRI’s[6]. A more recent randomized control trial found BLT, either alone or in combination with an SSRI, was more effective for treating nonseasonal major depression as the SSRI alone or the placebo[7]. Providing higher light levels earlier in the day helps improve depressive symptoms. The above illustrate mechanistically how this is happening: light directly and indirectly, impacts mood, and we need high light levels over a period of time to adequately saturate the system. Studies looking at lighting interventions for the circadian system also find improved mood, alertness, vitality, and even reduced depression[8–11].

These circadian lighting interventions do not provide as intense a light level as BLT, but the overall increase in light level in the environment for a longer duration seems to have a similar effect. Indeed, a meta-analysis of clinical trials looked at various BLT clinical trials and found that providing a Circadian Stimulus (CS) of >0.57 was effective[12]*.

It’s as though we experience a kind of year-round SAD that is induced by the biologically low light levels of our buildings. I think of it as Indoor Light Induced Depression (my own term, not a real diagnosis). In the same way that circadian disruption, even low-grade but chronically, can erode people’s health in different ways depending on their genetics and lifestyle, biologically murky light levels can undermine our mental health, affecting individuals by differing degrees.

Improving Light to Nourish Our Mental Health

The National Institute of Mental Health (may it, and science funding, survive DOGE), estimates that about 23% of Americans suffer from mental health challenges[13]. The amount of Americans experiencing depression is climbing[14]. In 2019, before the pandemic, the CDC did a survey and found that about 18% of people had experienced depression in the last 2 weeks before the survey alone[15]. Light, of course, is not the direct reason for this. Societal changes that are fracturing our real-person social networks, increased antagonism, and loneliness are reasons experts often point to.

Still, Americans spend an average of 90% of their day indoors. Given that light appears to be fundamental to our mental health, it makes me wonder if Indoor Light Induced Depression might be an eroding force that exacerbates the mental health crisis. Maybe by improving lighting in our buildings for our physiology, we can help nourish society’s mental health to reduce the overall severity, a kind of metaphorical high tide that lifts all boats.

This article is proudly researched and written by myself without the use of AI.

* I’ll talk more about CS in future articles, but briefly, CS accounts for light level, spectrum, and duration of exposure. An hour or less of outdoors daylight exposure can provide a CS of 0.57; so can about 4 or more hours of 250 lux of 3500K CCT LED light to the eyes (a modestly higher than typical light level in offices).

List of Citations

1.               Dr. Samer Hattar: Timing Light, Food, & Exercise for Better Sleep, Energy & Mood | Huberman Lab #43.; 2021. Accessed March 14, 2022. https://www.youtube.com/watch?v=oUu3f0ETMJQ

2.               Fernandez DC, Fogerson PM, Ospri LL, et al. Light affects mood and learning through distinct retina-brain pathways. Cell. 2018;175(1):71-84.e18. doi:10.1016/j.cell.2018.08.004

3.               Maruani J, Geoffroy PA. Multi-Level Processes and Retina–Brain Pathways of Photic Regulation of Mood. J Clin Med. 2022;11(2):448. doi:10.3390/jcm11020448

4.               Bedrosian TA, Nelson RJ. Timing of light exposure affects mood and brain circuits. Transl Psychiatry. 2017;7(1):e1017. doi:10.1038/tp.2016.262

5.               LeGates T, Altimus C, Wang H, et al. Aberrant light directly impairs mood and learning through melanopsin-expressing neurons. Nature. 2012;491(7425):594-598. doi:10.1038/nature11673

6.               Golden RN, Gaynes BN, Ekstrom RD, et al. The efficacy of light therapy in the treatment of mood disorders: a review and meta-analysis of the evidence. Am J Psychiatry. 2005;162(4):656-662. doi:10.1176/appi.ajp.162.4.656

7.               Lam RW, Levitt AJ, Levitan RD, et al. Efficacy of Bright Light Treatment, Fluoxetine, and the Combination in Patients With Nonseasonal Major Depressive Disorder: A Randomized Clinical Trial. JAMA Psychiatry. 2016;73(1):56-63. doi:10.1001/jamapsychiatry.2015.2235

8.               Boubekri M, Cheung I, Reid K, Wang CH, Zee P. Impact of Windows and Daylight Exposure on Overall Health and Sleep Quality of Office Workers: A Case-Control Pilot Study. J Clin Sleep Med JCSM Off Publ Am Acad Sleep Med. 2014;10:603-611. doi:10.5664/jcsm.3780

9.               Boubekri M, Lee J, MacNaughton P, et al. The Impact of Optimized Daylight and Views on the Sleep Duration and Cognitive Performance of Office Workers. Int J Environ Res Public Health. 2020;17(9):3219. doi:10.3390/ijerph17093219

10.            Figueiro MG, Plitnick BA, Lok A, et al. Tailored lighting intervention improves measures of sleep, depression, and agitation in persons with Alzheimer’s disease and related dementia living in long-term care  facilities. Clin Interv Aging. 2014;9:1527-1537. doi:10.2147/CIA.S68557

11.            Figueiro MG, Steverson B, Heerwagen J, et al. The impact of daytime light exposures on sleep and mood in office workers. Sleep Health J Natl Sleep Found. 2017;3(3):204-215. doi:10.1016/j.sleh.2017.03.005

12.            Zhou L, Hou D, Wang Y, Zhou S, Lin Y. High circadian stimulus lighting therapy for depression: Meta-analysis of clinical trials. Front Neurosci. 2022;16:975576. doi:10.3389/fnins.2022.975576

13.            Mental Illness - National Institute of Mental Health (NIMH). Accessed March 18, 2025. https://www.nimh.nih.gov/health/statistics/mental-illness

14.            Inc G. U.S. Depression Rates Reach New Highs. Gallup.com. May 17, 2023. Accessed March 18, 2025. https://news.gallup.com/poll/505745/depression-rates-reach-new-highs.aspx

15.            Products - Data Briefs - Number 379 - September 2020. September 24, 2020. Accessed March 18, 2025. https://www.cdc.gov/nchs/products/databriefs/db379.htm