Increase in the left hippocampal dentate gyrus head volume after a 4-week bright light exposure in healthy participants: A randomized controlled study.

Introduction: Bright light therapy (BLT) has been used for treating seasonal affective disorder, depression and bipolar depression. However, it's precise mechanism remains unclear. Bright light exposure (BL) induces neurogenesis in the adult rat hippocampal dentate gyrus (DG). We hypothesized that BL may induce neurogenesis in the human DG as well.

Method: A 4-week randomized controlled trial study was conducted, where healthy participants were randomly assigned to a BL group (10,000 lux) or dim light exposure group (DL group; 50 lux). Magnetic resonance imaging was performed at baseline and after 4 weeks. Longitudinal hippocampal subfield segmentation was generated via the FreeSurfer 7.1.1 hippocampal subfields module to evaluate volume of bilateral granule cell and molecular layer of the DG-head and -body.

Results: Our final sample size was 20, which consisted of BL group (n = 10) and DL group (n = 10). After age and sex adjustment, significant effects of time and group were detected in the left DG-head volume (p = 0.04). In the BL group, the left DG-head volume significantly increased (p = 0.004), whereas no significant volumetric change was observed in the DL group.

Conclusions: This study revealed that 4-week BL significantly increased left DG-head volume in healthy participants. Thus, neurogenesis might be induced by BL in the human DG, which is a completely new mechanism of BLT.