Association between nocturnal light exposure and melatonin in humans: a meta-analysis.
Level 3 - non-randomized controlled study
Meta-analysis of non-randomized observational/shift-work studies
PubMed 38123771 · doi:10.1007/s11356-023-31502-8
What was done
A systematic review and meta-analysis of 13 publications retrieved from PubMed and Web of Science evaluated the relationship between nocturnal light exposure (primarily in night shift workers) and melatonin levels compared to non-exposed or day shift controls. Outcomes pooled using mean differences (MD) and 95% confidence intervals (CI) included morning urinary 6-sulfatoxymelatonin (aMT6s), 24-hour urinary aMT6s, nocturnal urinary aMT6s, and blood melatonin concentrations.
What was found
Nocturnal light exposure was associated with statistically significant decreases across all assessed melatonin parameters: - Morning urinary aMT6s: significantly lower in the exposed group (MD = -3.69, 95% CI: -5.41 to -1.98, P < 0.0001; I² = 42%, P = 0.13). - 24-hour urinary aMT6s: significantly lower in night shift workers vs. day shift workers (MD = -3.38, 95% CI: -4.27 to -2.49, P < 0.00001; I² = 0%). - Nocturnal urinary aMT6s: significantly inhibited (MD = -11.68, 95% CI: -15.70 to -7.67, P < 0.00001; I² = 0%). - Blood melatonin: significantly suppressed (MD = -11.37, 95% CI: -15.41 to -7.33, P < 0.00001; I² = 0%).
Why it matters
This review provides quantitative evidence that night shift work and nighttime light exposure reliably suppress both circulating and excreted melatonin markers in humans, reinforcing the biological plausibility of circadian rhythm disruption in shift workers.
Limits
The abstract does not state the total number of participants, baseline demographic details, measurement units, or light exposure characteristics (such as lux, spectrum, or duration). Because the included studies are observational workplace cohorts rather than randomized controlled exposures, unmeasured confounders like daytime sleep quality, dietary factors, and chronotype cannot be ruled out.
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