Written by: Jack Quint | Graphics and editing : Adam Tzur | Published: September 30, 2021 1Updated October 4, 2021
How Poor Sleep Affects Weight Loss
Now that we have discussed a thorough background on the different sleep cycles and processes that occur overnight, we can dive into the main topic of this article: how exactly does sleep duration affect fat loss?
Sleep deprivation slows fat loss and burns muscle instead
We found that sleep restriction resulted in less loss of fat but greater loss of lean mass (...). - Wang et al. 2018 (23)
Two studies have looked at the effects of sleep restriction in an energy deficit on body composition (fat mass and fat-free mass) in adults [24; 23]. In these studies, all participants lost body weight. However, the sleep deprived participants lost more fat-free mass (i.e muscle and water).
Lack of sleep makes you eat more and move less
The basics of energy balance
Energy balance is how many calories you eat minus the calories you burn. This is also known as Calories In, Calories Out (CICO).
If you are eating more calories than you burn, then you will gain weight. If you expend more calories than you eat, you will lose weight.
Factors that affect calories in:
- Hormones
- Hunger
- How much food you eat
- Calories absorbed
Factors that affect calories out:
- Calories burned at rest (basal metabolic rate)
- Calories burned during exercise
- Calories burned outside of exercise (i.e. leisure activities)
- Calories burned when metabolizing food
Sleep affects energy balance
Overall, sleep restriction makes you more passive, thus expending less energy.
There are multiple lines of evidence that suggests that sleep restriction has a negative effect on energy expenditure. First, sleep restriction may reduce energy expenditure by decreasing physical activity due to tiredness and fatigue. In a crossover study on healthy adults, subjects had either a regular 24 hour sleep-wake cycle or a 24 hour period of complete wakefulness [2].
The researchers found that after a night of complete sleep restriction, subjects recorded a reduced resting and postprandial energy expenditure of 5% and 20%, respectively [2].
Other studies have found that sleep loss is associated with fatigue that results in not only less physical activity, but also less non-exercise activity thermogenesis, which has shown to have a profound effect on an individual’s daily energy expenditure [31].
It’s important to note though that not all studies find that sleep restriction has a profound effect on energy expenditure.
For example, in a 2010 randomized crossover study, 12 healthy men were restricted to either 4 or 8 hours a sleep [1]. Interestingly, physical activity was higher for the sleep-restricted group, even though their sleepiness was higher [1].
Some researchers have concluded that energy expenditure is not the primary component linking sleep deprivation to excess body fat, and is rather influenced by increased hunger and calorie intake [29; 31].
Sleeping Longer Speeds Up Weight Loss
Sleep extension favors weight loss in adolescents under caloric restriction (...) - Moreno-Frias et al., 2020 (41)
The current evidence suggests that increasing sleep duration and quality is beneficial for losing body fat. This might be because you are less likely to overeat, the longer you sleep [43].
Three studies have found that sleeping longer is associated with reduced fat mass (54, 43, 41).
Chaput (43) found that sleeping 1 hour longer was associated with a 0.7 kg decrease in fat mass. In Jåbekk (54), participants who trained and got sleep education lost 1.8 kg of fat. The control group who only trained without sleep education, did not lose fat.
Another study looked at the effects of sleep extension on body composition and metabolic conditions in obese adolescents [41]. 52 subjects received a diet with daily 500 calorie restriction, and were randomly assigned to either a sleep extension or control group. The sleep extension group aimed to increase their sleep by a total of 1 hour per night.
The researchers found that both groups lost weight, but the sleep-extension group improved weight loss and waist circumference to a significantly greater extent. Additionally, sleep extension improved inflammation and other metabolic conditions [41].
Collectively, these studies provide evidence that sleep extension may aid fat loss.
Methodology and Study Quality
Our Systematic Search Strategy
We researched sleep and fat loss using the Europe PMC database.
We used the following search string: (“sleep”) AND ("calorie restriction" OR "caloric restriction" OR "diet" OR "caloric deficit") AND (“fat mass” OR “body fat” OR “adipose tissue” OR “fat tissue” OR "weight loss") NOT ("disease" OR "cancer" OR "pain" OR "ventricular" OR "alcohol" OR "covid" OR "rat" OR "mouse" OR "mice" OR "animal" OR "insect" OR "patient").
Date range: we included all studies that were published up until 17.05.2021.
We reviewed the references of included studies to identify more studies. Additional studies were found in the “similar articles” section on the journal websites.
Study Outcomes
The primary outcomes we assessed from the available studies were the effect of sleep duration on fat mass, body mass, or BMI.
Our secondary outcomes included the effect of sleep duration on hormonal secretion, metabolism, energy expenditure, and appetite/calorie consumption.
Inclusion Criteria
Studies that were included in our analysis had to meet all of the following criteria:
- Were carried out in humans
- Subjects ranged in age from 18 years or older
- Was a narrative or systematic review analyzing sleep parameters on weight loss or body composition
- Assessed any of the primary or secondary outcomes
Exclusion Criteria
Additionally, studies from our systematic search were excluded for any of the following reasons:
- Consisted of animal, in vitro, disease or case studies
- Had fewer than 10 participants
- For experimental trials investigating primary outcomes: duration shorter than 2 weeks
Study Inclusion And Data Extraction
The systematic search yielded 673 studies. 29 studies were included after first-pass inclusion. These studies were scanned and 17 additional studies were extracted from the references. A second-pass exclusion was performed and 3 studies were removed.
This left us with a grand total of 43 included studies for the review. 6 additional references were included for other sections of the article.
The Strengths Of The Literature
Study length
One of the advantages of observational studies is that they are able to assess individuals over different time periods. Many of the studies in our review were several months long, sometimes upwards of multiple years. When we are able to assess longitudinal changes in a given variable over time, we can have more confidence in the data.
Strong mechanistic rationale
Although there aren’t many RCT’s looking at the effects of different sleep durations on body composition, many published studies have demonstrated mechanistically why sleep restriction likely limits fat loss. This includes many of the secondary outcomes we assessed in our review: decreased energy expenditure, increased appetite/food consumption, and altered hormones/metabolism.
Large sample sizes
Many studies in our review report findings from several hundred or thousands of participants, which can increase our confidence and the ecological validity of these findings. Larger sample sizes provide a smaller margin error by limiting the effects of outliers in a given investigation.
The Most Common Study Limitations
Study designs
Although observational and cross-sectional studies can enroll a large number of participants, their main flaw is that they cannot establish cause-and-effect between variables, and rather can observe general trends and relationships. To further investigate a causative relationship between sleep duration and fat loss, we’ll need additional randomized controlled trials, and eventually meta-analyses.
Lack of exercise or nutrition intervention
We’ve seen in thousands of studies over the years that exercise and nutrition can have a profound effect on body composition. Specifically, the inclusion of resistance training and an elevated protein intake have shown to help maintain FFM during an energy deficit. Future studies looking at the effect of sleep duration on multiple compartments of body composition (i.e. fat mass and fat-free mass) should aim to include an exercise and nutrition protocol to stimulate FFM retention.
Body composition measurements
Within the RCTs that we have which have measured body composition during a sleep intervention, body composition measurements are often assessed using a 2-compartment (fat mass and fat-free mass or 3-compartment (fat mass, bone mineral, other fat-free mass). Additionally, body composition is typically assessed with DEXA. Even though this is considered the “gold standard” body composition method, fat-free mass is largely confounded by hydration status. This means that it is not the ideal measure of skeletal muscle mass, compared to a validated method like ultrasound or MRI.