Lead. A large web-based survey in Japan has found that underweight women in their 20s are more likely to report insomnia symptoms, but the same link does not appear in adolescent girls or in women over 30. The finding represents one of the first efforts to document an age-specific relationship between underweight and sleep problems in a general population of females of reproductive age. Published July 11 in Sleep Health, the study analyzed survey responses from more than 5,600 Japanese females aged 12 to 49 years, drawn from a nationwide web panel.
What they found. Researchers led by Momoko Kayaba of the University of Tsukuba’s International Institute for Integrative Sleep Medicine (WPI-IIIS) conducted a cross-sectional, web-based survey from November to December 2023. Valid responses were obtained from 996 adolescent girls aged 12 to 18 years and 4,689 adult women aged 18 to 49 years.
Underweight was defined using body mass index thresholds standard in Japan. Among adolescent girls, 14.7% were underweight. Among adult women, the proportion was notably higher at 23.5%.
The survey assessed two distinct sleep-related outcomes. The first was a sleep debt pattern accompanied by daytime sleepiness, capturing behavior where insufficient sleep accumulates over time. The second was self-reported insomnia symptoms, reflecting difficulty falling asleep, staying asleep, or waking too early. The researchers used logistic regression models that adjusted for a range of sociodemographic and lifestyle factors, including age, education, employment status, smoking, alcohol use, and physical activity.
Among adolescent girls, underweight showed no statistically significant association with either sleep outcome. The picture was different for adult women. Underweight was significantly associated with self-reported insomnia symptoms, but this connection was confined to women in their 20s. No association was observed between underweight and the sleep debt pattern with daytime sleepiness in any age group. The researchers also detected a statistical interaction between age and underweight, meaning the relationship between the two weakened progressively as women grew older.
The team also explored whether the results could be explained by differences in lifestyle or health behaviors. After adjusting for these potential confounders, the association in the 20s age group remained statistically significant, suggesting the link is not simply an artifact of lifestyle differences between younger and older underweight women.
Why it matters. Underweight is a well-recognized health concern among individuals of reproductive age, associated with menstrual irregularities, reduced bone mineral density, fertility problems, and complications during pregnancy. While extensive research has examined the relationship between obesity and sleep disorders, far less attention has been paid to the opposite end of the weight spectrum. The findings suggest that young adult women who are underweight may represent a group warranting clinical attention for insomnia symptoms, even though the same pattern does not hold for adolescents or women in their 30s and 40s.
The age specificity of the finding is notable. Sleep and metabolic regulation share overlapping neuroendocrine pathways, including the hypothalamic-pituitary-adrenal axis and growth hormone signaling. The authors propose that these pathways may operate differently across developmental stages, with the twenties representing a window of particular vulnerability. The results point to the possibility that sleep assessment could be incorporated into broader health strategies aimed at supporting underweight young women, though the researchers caution that further work is needed to establish what drives the connection.
Limits. The study has several limitations that the authors acknowledge directly. As a cross-sectional survey, it captures a single point in time and cannot establish cause and effect. The sleep measures were self-reported through questionnaires rather than validated objective tools such as actigraphy or polysomnography, which may introduce reporting bias. The sample was drawn from a web-based panel in Japan, which may limit generalizability to other populations or to individuals without internet access. Residual confounding from unmeasured factors, including mental health conditions, eating behaviors, dietary patterns, and socioeconomic status, cannot be ruled out. The authors specifically call for further studies using validated sleep measures and longitudinal designs to confirm the findings and clarify the underlying mechanisms.
Bottom line. Underweight is associated with self-reported insomnia symptoms among adult women in their 20s, but not among adolescents or older women, according to this large Japanese survey. The finding identifies a specific age window in which sleep problems may merit attention as part of comprehensive approaches to underweight in young women. The results also underscore that the relationship between body weight and sleep is not uniform across the lifespan.
Source. Kayaba M, Ogata H, Yumen Y, Takayama Y, Kiyono K, Nose-Ogura S, Nagai N. Sleep problems and underweight among adolescent girls and adult women aged 12-49 years: A cross-sectional, web-based survey in Japan. Sleep Health. Published online July 11, 2026. doi: 10.1016/j.sleh.2026.06.005. PMID: 42431813.