Fig. 2

Nonlinear dose–response relationship between sleep duration and thyroid hormones. Locally weighted scatterplot smoothing (LOESS) curves with 95% confidence intervals demonstrate the nonlinear association between sleep duration and serum thyroid hormones (TSH, T3, T4). Both short (≤ 6 h) and long (≥ 9 h) sleep durations correspond to altered hormone levels, with an inflection point near 7 h indicating a U-shaped pattern.