Bright light therapy (BLT) has been considered a potential treatment for depression symptoms, yet the mechanisms by which light impacts mood are still under investigation. A recent study assessed sleep regularity as a mediating factor in the relationship between bright light exposure and depression symptoms.

Methods

This cross-sectional study analysed data from the 2011-2014 National Health and Nutrition Examination Survey (NHANES), representing the noninstitutionalised U.S. population. Nonpregnant participants aged 18 years or older with valid light and actigraphy measurements were included. The study followed STROBE guidelines and utilised publicly available, deidentified data approved by the Ethics Review Board of the Centers for Disease Control and Prevention. All participants provided written informed consent.

Measurements

• Bright Light Exposure: Measured using time above lux threshold (TALT1000) captured by wrist-worn GT3X+ ActiGraph.
• Sleep Regularity Index (SRI): Assessed using actigraphy data.
• Depression Symptoms: Evaluated with the Patient Health Questionnaire-9 (PHQ-9), where scores range from 0 to 30, with higher scores indicating greater frequency of symptoms.

The analysis, conducted between March 2023 and May 2024, applied weighted linear, quasi-Poisson, and logistic regression models to test associations between TALT1000, SRI, and PHQ-9 scores. Adjustments were made for age, sex, race and ethnicity, season, employment, physical activity, sleep duration, body mass index (BMI), and serum cotinine levels.

Results

The study included 6,639 participants (mean age 49.41 years; 52.2% female). The demographic breakdown was:

• 8.3% Mexican American
• 4.4% non-Hispanic Asian
• 11.3% non-Hispanic Black
• 67.7% non-Hispanic White
• 5.7% other Hispanic
• 2.6% other or multiracial

Key Findings

• Gender Differences: Male participants exhibited greater TALT1000 and fewer depression symptoms than female participants.
  
• Bright Light and Depression: Greater TALT1000 was associated with lower depression symptoms (β = -0.19; 95% CI, -0.34 to -0.04). However, this association disappeared after adjusting for SRI (β = -0.11; 95% CI, -0.26 to 0.04; P = .13).
  
• Bright Light and Sleep Regularity: Longer duration in bright light was linked to more regular sleep (β = 1.60; 95% CI, 0.95-2.25).
  
• Sleep Regularity and Depression: More regular sleep correlated with a lower depression score (β = -0.05; 95% CI, -0.06 to -0.03) and reduced odds of mild or more severe depression (OR = 0.98; 95% CI, 0.97-0.99).
  

Exploratory Analyses

• Vitamin D: Higher vitamin D levels were associated with greater TALT1000 and more regular sleep but did not show a significant relationship with depression symptoms.

The study highlights the intricate relationship between bright light exposure, sleep regularity, and depression symptoms. While longer exposure to bright light appears to enhance sleep regularity and subsequently reduce depression symptoms, the direct impact of bright light on depression diminishes when sleep regularity is accounted for. These findings suggest that sleep regularity might play a crucial role in the mood-boosting effects of bright light exposure.

Source: JAMA Network