Susan Kohl Malone, assistant professor and lead author of the investigation, stressed that sleep timing deserves as much attention as sleep duration. "When we fall asleep—not just how much we sleep—seems to be an important and overlooked factor for weight loss and metabolic health," she stated.
Previous research has linked inadequate sleep, including sleeping for less than seven hours a night or following irregular sleep patterns, with weight gain and poor glucose regulation. Insufficient sleep can increase stress hormones such as cortisol, contributing to insulin resistance. Changes in appetite-regulating hormones can also increase hunger. Later bedtimes are frequently associated with late-night eating, when the body's insulin sensitivity and glucose tolerance are at their lowest levels.
Against this backdrop, the research team investigated the role of sleep health in diabetes prevention. The researchers conducted a randomised controlled trial involving 196 adults aged between 21 and 64, all of whom reported sleeping less than 6.5 hours each night. Participants were divided into two groups: one followed an eight-week lifestyle programme, while the other received the same programme along with additional sleep strategies.
Both groups attended weekly video sessions covering healthy eating, physical activity and stress management, with the guidance tailored towards diabetes prevention. Participants in the lifestyle-plus-sleep group also received additional coaching focused on sleep hygiene, reconditioning their perception of the bed as a cue for sleep and gradually shifting their bedtimes to increase overall sleep duration. They maintained daily sleep diaries throughout the eight-week trial.
Researchers assessed participants before and after the intervention using continuous glucose monitors to measure blood sugar levels and wrist monitors to track sleep patterns.
The findings showed that neither programme produced changes in glucose levels. However, participants in both groups experienced improvements in sleep and weight loss. Those receiving the additional sleep coaching also reported better sleep quality and reduced daytime sleepiness.
A notable finding was the shift towards earlier bedtimes among participants in the lifestyle-plus-sleep group. When researchers analysed the results across all participants, they found that falling asleep earlier was associated with greater weight loss, regardless of total sleep duration.
According to the analysis, participants lost approximately 1.3 pounds over the eight-week period for every hour earlier they fell asleep. This association suggests that sleep timing may influence weight loss independently of the total number of hours spent sleeping.
The researchers proposed several possible explanations for the relationship. Staying awake late may increase exposure to artificial light, heighten sympathetic nervous system activity and raise levels of ghrelin, commonly known as the "hunger hormone". These changes could contribute to increased appetite and potentially affect weight management.
Malone acknowledged that some expected outcomes did not materialise during the trial. "While we expected that extending sleep would improve glucose levels and weight loss in adults with prediabetes who sleep for shorter periods, this wasn’t the case in our study. However, our findings do raise the possibility that earlier sleep onset, independent of sleep duration, could be a promising target for improving metabolic health."
The findings highlight the potential importance of sleep timing in diabetes prevention programmes. Although the intervention did not produce measurable changes in blood sugar levels, the association between earlier sleep onset and greater weight loss points to bedtime as a factor that may warrant greater attention in efforts to improve metabolic health among people with prediabetes.
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