What This Means
This research suggests that the timing of sleep — specifically when during the night a person sleeps (sleep midpoint) and how much that timing shifts between weekdays and weekends (social jetlag) — is not meaningfully associated with the risk of developing cancer. The study followed nearly 145,400 adults in the United States over several years, tracking who developed cancer through official state cancer registries, and found no statistically significant links between these sleep timing measures and either overall cancer risk or breast cancer risk specifically.
The study used a large, prospective design, meaning participants were enrolled before they developed cancer and followed over time, which is considered a strong approach for studying risk factors. Sleep information was collected through self-reported surveys in 2015, and researchers accounted for many other factors that could influence cancer risk, such as demographics, socioeconomic status, health conditions, and lifestyle behaviors. Over the follow-up period ending in 2020, more than 5,500 cancer cases were identified.
This research suggests that, at least based on self-reported sleep timing data, people who sleep at different clock times or who experience significant shifts in their sleep schedule between work and non-work days are not at measurably higher or lower risk for cancer compared to those with more typical or consistent sleep timing. These findings add important null evidence to a field where prior research on sleep and cancer has largely focused on sleep duration and quality rather than timing.