Research suggests that...
- Sleep recovers your body and mind. Hormones are key to this regeneration.
- Sleep deprivation will put your body in a catabolic state. This means you may gain fat and lose muscle mass.
- If you are on a diet, sleep longer to speed up weight loss and prevent muscle loss.
The full science-based article continues below the infographic.
What Happens When We Sleep
Outside of training and nutrition, recovery should be your top priority for building muscle, gaining strength, or losing body fat.
Getting adequate sleep at night is essential for recovery from both a performance and health perspective. In this article, we’ll show you the scientific data on sleep duration and body composition.
Sleep Stages And Sleep Cycles
There are two primary types of sleep: rapid eye movement (REM) and non rapid eye movement (non-REM). While REM is just a single sleep stage, non-REM contains three stages- each of which have their own specific functions towards recovery.
Here’s a brief summary of each of the four sleep stages from sleep onset to deep sleep .
- Stage 1 (non-REM): this happens during the first few minutes of falling asleep. Heart rate and breathing begin to slow down.
- Stage 2 (non-REM): a period of light sleep before you enter deep sleep, typically lasting for around 25 minutes of the sleep cycle.
- Stage 3 (non-REM): the slow-wave deepest sleep cycles. It is during this time that the greatest amount of recovery and regeneration takes place, leaving you refreshed in the morning.
- Stage 4 (REM): the final stage of sleep. Eye movement and brain activity significantly increase, and this is when most dreams will occur.
Combined, these stages make up one sleep cycle. This cycle repeats itself throughout the night and lasts approximately 90 minutes each time.
How sleep helps the body recover
Getting a good night of sleep helps us feel refreshed and recovered in the morning, and that doesn’t just happen magically. During each sleep cycle, a variety of biological processes take place that support our health.
Here are some key processes that happen overnight :
- Cellular and tissue restoration: throughout the day, we perform a wide variety of activities that stresses our body. During sleep, muscles and tissues are repaired.
- Improved brain function: sleep clears out waste and improves cognitive function. Sleep deprivation can negatively affect learning and memory.
- Energy conservation: when we sleep, we greatly reduce our energy expenditure. At the same time, we are not consuming energy, leaving us at a relative energy balance. This helps facilitate recovery and allows us to save our energy for when we need it during the day.
The role of hormones
Sleep is a crucial time for the release of hormones in the body. Many of these hormones are responsible for aiding in muscle repair, the regulation of appetite, or neurotransmitters that influence mood and stress levels.
Some of the most important ones are described below.
|Name Of Hormone||↑↔↓ During Sleep?||Primary Functions|
|Ghrelin||↓ Normal Sleep
↑ Sleep Deprivation
|Produced and mainly secreted by the stomach
Referred to as the “hunger hormone” for stimulating appetite
Stimulates the release and circulation of growth hormone
|Leptin||↑ Normal Sleep
↑ Sleep Deprivation
|The “antithesis” of ghrelin- sends signals to the brain to alter food intake and energy expenditure
Produced by fat cells and helps us maintain a healthy body weight
|Growth Hormone||↑ Normal Sleep
↔ Delayed Compensation Under Sleep Deprivation
|Produced by the anterior pituitary gland
Regulates metabolism and promotes healthy bone and tissue growth
Aids in cell repair/reproduction
|Cortisol||↓ Normal Sleep
↑ Sleep Deprivation
|One of the primary stress hormones; regulates blood pressure and the sleep-wake cycle
Elevates in a sympathetic state and decreases at rest
|Catecholamines||↓ Normal Sleep
↑ Sleep Deprivation
|Include epinephrine, norepinephrine, and dopamine
Produced by the adrenal glands along with the brain
Part of the body’s stress (i.e. fight-or-flight-response)
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).
|Author||Study facts||Intervention||Key Findings|
|Nedeltcheva, 2010 (24)||10 subjects
|Moderate calorie reduction with either 8.5 or 5.5 hours in bed each night||Both groups lost body mass and body fat.
Sleep restriction (5.5 hours) significantly decreased the amount of body fat lost (by 55%) and significantly increased the amount of fat-free mass lost (by 60%)
|Wang, 2018 (23)||36 subjects
|Normal-sleep group maintained their regular sleep habits
Sleep-restricted group instructed to decrease sleep duration during the week, but could sleep an ad libitum amount on the weekend (i.e. “catch-up)
|Both groups lost a similar amount of weight, lean mass, and fat mass
However, in the sleep restriction group, the total % of mass lost as fat mass was significantly lower (~17% compared to 80%), and the total % of mass lost of fat-free mass was significantly higher (~85% compared to ~17%).
When sleep deprived,
- the body produces more cortisol and less testosterone,
- muscle protein synthesis goes down, and
- muscle protein breakdown may go up
A single night of total sleep deprivation is sufficient to induce anabolic resistance (...) - Lamon et al., 2021 (56)
Hence, sleep deprivation is catabolic.
Finally, a 2015 meta-analysis examined the impact of sleep duration on body mass. The researchers found that restricting sleep led to either more weight gained or less weight lost, although it was not a statistically significant effect .
If you have physique goals, this is not good. You want to maintain as much muscle mass as possible during a diet, while burning as much fat as possible.
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:
- 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 .
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 .
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 .
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 . Interestingly, physical activity was higher for the sleep-restricted group, even though their sleepiness was higher .
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 .
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 . 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 .
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.
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.
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
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
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
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.
Systematically collected studies