What a Slightly Bright Bedroom Is Linked to in Over 100,000 Women

Drawing on more than 100,000 women taking part in a long-running UK health study, researchers asked a simple question about bedroom lighting, on a four-point scale from 'light enough to read' down to 'too dark to see your hand', and compared the answers against several measures of body size. Women who reported sleeping in the brightest bedrooms had a higher body mass index, larger waist circumference and higher waist-to-hip ratio than those who slept in complete darkness, and the pattern held even after accounting for other factors that could explain it, including physical activity, sleep duration and having young children in the home.

This is a large, well-powered study, but it is cross-sectional, meaning light exposure and body measurements were captured at the same point in time, so it cannot establish which came first or rule out other explanations entirely. The researchers themselves were careful to say the findings don't yet justify specific advice about curtains or nightlights. What the scale of the sample does offer is confidence that the association is real and worth taking seriously, adding human evidence to a mechanism, light disrupting circadian and metabolic signalling, that has been demonstrated more directly in animal studies.