Bright light exposure may improve seasonal depressed mood: more studies are needed

Regular morning exposure to bright light may improve seasonal depressed mood

This is the third post in a series on non-pharmacologic treatments of depressed mood. Previous posts provided brief reviews of the evidence for S-adenosylmethionine (SAMe) and folate. This post is offered as a concise discussion of bright light exposure therapy as a treatment of depressed mood. In addition to commenting on findings of individual studies I comment on the disparity in meta-analysis findings and the resulting debate on the efficacy of bright light therapy.

Mechanism of action is probably multi-factorial 

The mood enhancing benefits of bright light are probably related to many mechanisms of action that affect regulation of the synthesis of melatonin and neurotransmitters, especially the monamine neurotransmitters serotonin, dopamine and norepinephrine. Recent theoretical work suggests that the beneficial effects of light on the body and brain may be consistent with meridian theory in Chinese medicine (Cocilovo 1999).

Many studies support that exposure to bright light (10,000 lux) in the early morning, for one to two hours daily over several weeks (10,000 lux) has therapeutic effects in moderately or severely depressed patients. This effect is especially robust in patients who report recurring seasonal depressed mood changes–so-called seasonal affective disorder (SAD).

Small studies, short duration and methodological differences limit findings

Many studies report that morning exposure and evening exposure to bright light are probably equally effective in seasonal depressed mood however there are reports of insomnia with evening exposure. In addition to artificial full-spectrum bright light, exposure to natural sunlight, especially in the early morning, also has a significant anti-depressant effect, and may reduce the length of hospital stays in severely depressed bipolar inpatients (Benedetti 2001). Findings of a randomized double-blind study (89 total subjects) suggest that bright light delivered transcranially by LED inserted into the ear canals may have both antidepressant and anti-anxiety benefits in individuals diagnosed with seasonal affective disorder (Jurvelin 2014).

Findings of a small randomized trial suggest that bright light therapy may be an effective alternative to antidepressants in pregnant depressed women (Epperson 2004). In another study depressed patients who exercised regularly improved more when exposed to bright light than ordinary room light (Partonen 1998). A study published in the Journal of Affective Disorders in 2018 found that individuals diagnosed with seasonal affective disorder randomized to bright full spectrum exposure vs narrow spectrum low intensity blue light reported equivalent improvements in depressed mood symptoms (Meesters 2018).

Meta-analyses report conflicting findings

Although the above studies and many other controlled studies published in the peer-reviewed medical journal literature have reported beneficial effects of bright light exposure on seasonal depressed mood–and in some cases non-seasonal depressed mood, the conclusions of meta-analyses are inconsistent. For example, in an early meta-analysis Golden et al (Golden 2005) reported on large therapeutic benefits (i.e., a large ‘effect size’) of bright light exposure vs sham. The authors concluded that bright light exposure or dawn simulation for seasonal depressed mood, and bright light exposure (but not dawn simulation) for non-seasonal depression has comparable efficacy to conventional antidepressants (Golden 2005).  However, a more recent meta-analysis using more stringent selection criteria found only equivocal evidence in support of bright light exposure for depressed mood (Martensson 2015) arguing that findings of most studies are limited by small size, short duration and methodological differences in terms of light intensity, exposure duration, light source and wavelength examined, etc. Martensson et al also pointed out that selection bias in the Golden et al meta-analysis had led to unsubstantiated claims on the basis of 3 very small studies reporting positive outcomes.

Side effects can include headaches, insomnia and nausea

Some patients exposed to bright morning light 10,000 lux on a regular basis report transient side effects including mild jitteriness or headaches, and mild nausea. Sporadic cases of hypomania have been reported, especially in winter depressives or Bipolar patients exposed to early morning bright light. Almost two thirds of patients who use bright light exposure therapy in the evening report insomnia. Because of the risk of insomnia with evening bright light exposure, everyone who considering the use of bright light exposure to treat depressed mood should do so in the morning only.

Bottom line

Despite decades of research on full spectrum bright light (and more recently, dim blue light) exposure as a treatment of seasonal depressed mood, there is still no consensus among researchers on the antidepressant efficacy of this therapy. The wide disparity in reported outcomes reflects methodological differences in study designs, and suggests that factors that influence response to bright light exposure may not yet be adequately characterized. These factors may include the relative intensity of light, exposure duration, timing of exposure, frequency of exposure and wavelength. In addition, variation between individuals undergoing bright light therapy such as genetic, epigenetic and other poorly characterized inter-individual differences may help explain the disparity in response. Large prospective studies identifying and controlling for these factors are needed to elucidate the complex mechanisms of light therapy and to clarify its potential antidepressant effects.

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