A large study of over 3,600 Spanish adults found that people who sleep more than 8 hours per night lost less weight during a weight loss program compared to those who sleep 7-8 hours. People who slept too much were also more likely to quit the program early. Interestingly, sleeping less than 7 hours didn’t seem to hurt weight loss as much as sleeping too much did. This suggests that getting the right amount of sleep—not too much, not too little—might be important for successfully losing weight.

The Quick Take

  • What they studied: Whether the amount of sleep people get at night affects how much weight they lose when following a weight loss program
  • Who participated: 3,628 adults from Spain who were overweight or obese, with an average age of 41 years; about 78% were women. They participated in a 25-week weight loss program that focused on changing eating and exercise habits
  • Key finding: People who regularly slept more than 8 hours per night lost about 0.5% less weight on average than those who slept 7-8 hours. They were also 21% less likely to lose at least 5% of their body weight and 25% less likely to lose weight quickly
  • What it means for you: If you’re trying to lose weight, aiming for 7-8 hours of sleep per night may help you succeed better than sleeping longer. However, this study shows a connection, not proof that extra sleep causes weight loss problems—other factors could be involved

The Research Details

This study looked back at information from a larger weight loss program called ONTIME that took place over 25 weeks. Researchers asked participants how much they typically slept at the beginning of the program, then divided them into three groups: short sleepers (less than 7 hours), normal sleepers (7-8 hours), and long sleepers (more than 8 hours). They then tracked how much weight each group lost and whether they stayed in the program.

The researchers used statistical methods to compare the groups while accounting for other factors that might affect weight loss, such as age, sex, starting weight, and which clinic people attended. This approach helps isolate the effect of sleep from other variables that could influence the results.

This type of study is called a secondary analysis because researchers used data that was already collected for another purpose, rather than designing a brand new study from scratch.

Understanding how sleep affects weight loss is important because many people struggle with both sleep problems and weight management. If sleep duration truly affects weight loss success, doctors and weight loss programs could use this information to help people set better sleep goals alongside diet and exercise changes. This study is stronger than previous research because it included many more participants and tracked them over several months in a real-world weight loss program.

This study has several strengths: it included a large number of participants (3,628), tracked them over a meaningful time period (25 weeks), and adjusted for multiple factors that could affect results. However, participants reported their own sleep duration rather than having it measured objectively, which could introduce some inaccuracy. The study was also conducted in Spain, so results may not apply equally to all populations. Additionally, this was a secondary analysis, meaning the original study wasn’t designed specifically to examine sleep and weight loss.

What the Results Show

Among the 3,628 participants, about 61% reported sleeping the recommended 7-8 hours per night, 24% slept more than 8 hours, and 15% slept less than 7 hours. People who slept more than 8 hours lost an average of 7.42% of their body weight, compared to 7.90% for those sleeping 7-8 hours—a difference of about 0.5%.

When researchers looked at meaningful weight loss (losing at least 5% of body weight), long sleepers were 21% less likely to achieve this goal. For rapid weight loss (losing at least 0.53 kg or about 1.2 pounds per week), long sleepers were 25% less likely to reach this pace. Additionally, people who slept too much were 21% more likely to drop out of the weight loss program entirely.

Interestingly, people who slept less than 7 hours per night did not show significantly worse weight loss results compared to the 7-8 hour group. This suggests that sleeping too much may be more problematic for weight loss than sleeping too little, at least in this study.

The researchers used advanced statistical methods to confirm these findings while controlling for age, sex, starting weight, and other factors that could influence weight loss.

The study found that the relationship between long sleep and reduced weight loss was consistent across different analyses. When researchers examined the rate of weight loss over time, long sleepers consistently showed slower progress. The increased dropout rate among long sleepers is particularly noteworthy because it suggests that people sleeping too much may become discouraged or face other challenges that make them quit the program.

Previous research has shown that both too little sleep and too much sleep can affect metabolism and appetite hormones, but studies have been small and produced mixed results. This study is one of the largest to examine this question in a real weight loss program. The finding that long sleep is more problematic than short sleep for weight loss is somewhat surprising and adds new information to the scientific understanding of sleep and weight management.

The main limitation is that participants reported their own sleep duration rather than having it measured with devices, which could be inaccurate. The study was conducted in Spain with mostly women (78%), so results may not apply equally to men or people from other countries. The study shows a connection between sleep and weight loss but cannot prove that extra sleep causes the weight loss problems—other unmeasured factors could be involved. Additionally, the study didn’t examine why long sleepers lost less weight, so we don’t know the exact mechanism.

The Bottom Line

If you’re trying to lose weight, aim for 7-8 hours of sleep per night rather than significantly more. This recommendation is based on moderate evidence from this large study. However, this should be combined with healthy eating and exercise—sleep alone won’t cause weight loss. If you naturally sleep more than 8 hours and feel well-rested, discuss with your doctor whether adjusting your sleep duration is appropriate for you.

This finding is most relevant for adults who are overweight or obese and are actively trying to lose weight. People who sleep more than 8 hours and are struggling with weight loss might benefit from examining their sleep patterns. However, some people may need more sleep due to medical conditions, medications, or other factors—this study doesn’t apply to everyone equally. If you have sleep disorders or medical conditions affecting sleep, consult your healthcare provider before making changes.

Weight loss is a gradual process. In this study, participants were tracked over 25 weeks (about 6 months). If you adjust your sleep duration, you might notice effects on your weight loss progress within several weeks, but meaningful changes typically take months to become apparent.

Want to Apply This Research?

  • Track your nightly sleep duration (in hours) alongside your weekly weight. Set a target of 7-8 hours per night and monitor whether weeks with consistent sleep in this range correlate with better weight loss progress compared to weeks with longer sleep.
  • If you’re currently sleeping more than 8 hours regularly, try gradually shifting your bedtime earlier by 15-30 minutes per week until you reach 7-8 hours. Use the app to log your sleep target and actual sleep, then note your energy levels and weight loss progress to see if the adjustment helps.
  • Create a monthly dashboard showing average sleep duration, total weight lost that month, and program adherence. Look for patterns between months when you maintained 7-8 hour sleep versus longer sleep periods to see if the relationship holds true for your personal weight loss journey.

This study shows an association between sleep duration and weight loss outcomes but does not prove that sleep duration causes changes in weight loss. Individual results vary based on genetics, overall health, diet, exercise, and many other factors. Before making significant changes to your sleep schedule or starting a weight loss program, consult with your healthcare provider, especially if you have sleep disorders, medical conditions, or take medications that affect sleep. This information is for educational purposes and should not replace professional medical advice.