Bright Light Therapy in Psychiatric Disorders: Mechanisms, Clinical Procedures and Evidence.

Light is the primary zeitgeber for circadian rhythms, and also through these mechanisms, is closely related to mood regulation. Bright light therapy (BLT) is a therapeutic intervention that specifically exploits this physiological mechanism. This review summarizes the clinical procedures of BLT, the mechanisms through which light influences circadian rhythms and mood, and the evidence supporting BLT in psychiatric disorders. BLT is administered by considering device distance, treatment duration, and light intensity. Through pathways originating in the retina and projecting to the Suprachiasmatic Nucleus (SCN), light might generate signals within the central nervous system that influence not only circadian regulation but also mood, via connections involving the limbic system, the lateral habenula, and interactions with the hormonal system. At the clinical level, the strongest evidence for BLT concerns seasonal affective disorder, but data also indicate antidepressant efficacy in major depressive disorder and bipolar disorder, with an excellent tolerability profile. Emerging evidence further suggests benefits for insomnia, and sporadic and heterogeneous findings have explored its potential role in other conditions. Future studies are needed to better define the role of BLT in additional psychiatric disorders and in specific symptom domains that may not adequately respond to standard treatments, such as sexual dysfunction.