Out of all the excess body fat we can have, belly fat is the one many people hate the most. But, beyond looks, belly fat is also the most dangerous type of fat. It can also be some of the hardest to lose, and it can be caused by much more than a bad diet.
In this blog post, we’ll dive into what causes belly fat and how you can lose it in your sleep — literally. We explain how getting enough sleep and aligning this sleep, as well as your meals, with your circadian rhythm can not only stop you putting on more belly fat, it can help you lose it, too.
First up, what exactly is belly fat? Of course, it’s body fat that is stored around the belly area, but there are two different types.
Subcutaneous fat is what most of us think of when we think about body fat. It’s the fat you can see and feel that sits just under the skin.
Visceral fat, on the other hand, is stored deep inside the belly area around organs like the liver and intestines. This fat is much more dangerous as it’s associated with metabolic syndrome — which includes high blood pressure, obesity, high cholesterol, and insulin resistance. Having a lot of visceral fat increases your risk of stroke, heart disease, certain types of cancer, and type 2 diabetes. Although you may not be able to see this fat, it can still make your belly area protrude forward.
Belly fat can be caused by anything that makes you gain weight in general, but there are also lifestyle factors that can lead to more belly fat specifically. These include:
Let’s dive into those last two in more detail.
Not getting enough sleep has been linked to weight gain in general. For example, one meta-analysis looked at multiple studies and found sleep deprivation may lead to an increase in ghrelin — the hormone that makes you hungry — and a decrease in leptin — the hormone that suppresses appetite. It also usually leads to an increase in food intake and a decrease in exercise. This, of course, is a perfect storm for weight gain. But sleep deprivation can also make you gain belly fat specifically.
New research, published in 2022, found not getting enough sleep led to an increase in calorie intake, body fat, and belly fat. The researchers found people who slept four hours a night had a 9% increase in subcutaneous belly fat and an 11% increase in abdominal visceral fat, compared to those who slept 9 hours a night.
What’s more, the participants sleeping for four hours a night did so for only 14 nights, so it doesn’t take long for a lack of sleep to catch up with you.
Another study found those who slept for five hours or less a night were more likely to be obese and have a larger waist circumference than those who slept for seven to nine hours a night. In fact, those who slept five hours or less were 24% more likely to be abdominally obese than those who slept for seven hours a night.
Abdominal obesity is described as having a waist circumference of about 35 inches or more in men and 33 inches or more in women. Participants were also 25% more likely to be obese, and if you’re obese, belly fat is usually a problem, too.
Research shows people who regularly sleep for less than six hours a night tend to have a higher body mass index (BMI), fat percentage, and abdominal circumference than those who sleep for seven to eight hours a night.
And even just one hour of sleep makes a difference when it comes to belly fat. Research suggests those who sleep for six hours a night have more fat stored in their abdominal area than those who sleep for seven hours a night.
Not getting enough sleep can also lead to more belly fat simply because you have more cravings for unhealthy and processed foods when you’re tired, so you tend to have an unhealthy diet. This leads to carrying more fat overall, including around the belly.
But it’s not just the direct link between sleep and fat storage. One study looked at participants who slept for five hours a night for one week. The results showed this sleep deprivation significantly reduced insulin sensitivity, which could lead to insulin resistance.
When our bodies become resistant to insulin, the pancreas makes more and more of the hormone in order to get the glucose in our blood to be absorbed into our muscles and organs, where it’s needed for energy. But higher insulin levels can trigger the body into storing this glucose as fat, especially around the belly.
And it doesn’t take long for this to happen. One study found a single night of four hours of sleep can lead to increased glucose production by the body, decreased insulin sensitivity, and evidence of insulin resistance. If there’s excess glucose in our system that can’t be used as energy, it can be stored as body fat.
Not sleeping for long enough may also increase levels of the stress hormone cortisol in the evenings, which may decrease insulin sensitivity the next morning. Cortisol itself has been shown to trigger the body into storing fat, especially stomach fat.
Even disturbed sleep is associated with insulin resistance and a reduced insulin response to glucose — all of which can lead to an increase in body fat, especially around the belly.
Other studies show those who don’t get enough sleep or those who have disturbed sleep are more at risk of developing type 2 diabetes. Research has found those who sleep for six hours a night were 28% more at risk, those who find it difficult to get to sleep were 57% more at risk, and those who find it hard to stay asleep were 84% more at risk, compared to those who sleep eight hours a night.
Low sleep efficiency — the measure of how long you spend sleeping while in bed — has also been linked to obesity, and specifically to abdominal obesity.
Your circadian rhythm is your internal body clock that runs over a roughly 24-hour cycle. Sleeping and eating at different times each day, or at the wrong times for your body clock, can disrupt this cycle and lead to weight gain, which can lead to more fat being stored around the belly.
One study found those who had disrupted circadian rhythms were more at risk of abdominal obesity.
Being out of sync also messes with the hormones related to hunger and makes falling asleep harder — which leads to eating more and sleeping less, leading to weight gain. Even if you’re getting the right amount of sleep, just at the wrong times, hunger hormones ghrelin and leptin are still affected simply by being out of sync with your circadian rhythm.
Being out of sync is a problem for those who work night shifts, of course, but it’s actually much more common than you think. Even being just one or two hours out of alignment with your circadian rhythm impacts your weight.
Social jetlag is when your biological clock is out of sync with your social clock. This often happens when we stay up and wake up late on the weekends, which about 87% of adults do.
A 2015 study found people with more social jetlag had higher BMIs, more fat mass, and were more likely to be obese. Researchers found social jetlag was associated with obesity, even when they took into account how long people slept for.
Sleeping at different times each night has been linked to obesity — both because it may lead to sleep deprivation and because it often leads to out-of-sync eating patterns.
And sleep timing is just as important as sleep duration when it comes to your weight. Going to bed late and waking up late has been linked to obesity, as it shifts meal times to later in the day, and those with a later bedtime are more likely to skip breakfast and have late-night snacks.
One study found those who went to bed later ate more calories after 8 p.m. compared to those who went to bed earlier, which was associated with a higher BMI even when taking into account sleep duration and sleep time. Another study found eating breakfast before 8:30 a.m. led to lower blood sugar levels and less insulin resistance compared to eating it later.
As well as meal timing, you should consider which meals you eat the bulk of your calories in. For example, one study looked at two groups who ate the same amount of calories. The first group had higher-calorie breakfasts and lower-calorie dinners, and the second group did the opposite, eating more calories at dinner time. Those who ate more of their daily calorie intake at breakfast lost more weight and had a bigger reduction in waistline circumference. This group also had lower insulin, glucose, and ghrelin levels, and were more satisfied.
Beyond calories, spreading your meals over a large period of the day keeps your body in fat-making mode, instead of fat-burning mode. To make the switch, you need to give your digestive system enough time when it’s not processing food. This can be done by having your evening meal earlier, so that the window of time between your last meal and the next day’s breakfast is a little longer.
Intermittent fasting, or time-restricted eating, is when you reduce the window of time in which you eat your meals during the day. For example, if you have an eight-hour window, you’d only eat between say 10 a.m. and 6 p.m. There is some evidence to show intermittent fasting may help with weight loss and reduce the risk of obesity. We dive more into how intermittent fasting affects weight loss here.
If you want to burn belly fat specifically, it can be tempting to try to “spot reduce” fat loss in this area by doing a lot of crunches and ab workouts, for example. But that’s not the way fat loss works.
Most of the time, reducing your overall body fat will lead to a reduction in abdominal fat, as well as fat elsewhere. But you can’t pick and choose where you lose fat from in your body.
For example, one study had participants do ab workouts for six weeks and found there was no significant change in belly fat or abdominal circumference. They did improve their muscular endurance, though.
So, while you can't target fat loss to your belly specifically, there are things you can do to lose fat in general, and make sure you don’t put on additional belly fat.
Things like dietary supplements, detox green teas, and fat-burner drinks all promise to help you lose weight fast and get you a flat belly, but there is a more healthy way to do it.
We’ll dive into these two in more detail below.
Your sleep need is the genetically determined amount of sleep you need each night. Getting more sleep at night doesn’t just help with subcutaneous fat, it can help you not gain visceral fat, too.
One study found switching from sleeping six hours or less a night to sleeping seven to eight hours a night would lead to gaining almost 1 square inch less visceral fat. This was after BMI, exercise, and calorie intake were taken into account, too.
The RISE app can calculate your individual sleep need, so you know exactly what you should be aiming for each night.
As sleep deprivation can lead to gaining belly fat, you should also work to lower your sleep debt. This is the measure of how much sleep you owe your body over the last 14 nights. We recommend keeping sleep debt below five hours to feel and perform your best, but this will also help reduce the likelihood of sleep deprivation causing weight gain, and make it easier to lose stubborn belly fat.
If you find you have more than five hours of sleep debt, you can pay it back by:
It’s not just how long you sleep for, when you sleep and eat can also impact belly fat. Here’s what to do:
We’ve covered how disturbed sleep can lead to obesity and how low sleep efficiency can lead to belly fat. And this is where sleep hygiene can help.
Sleep hygiene is a set of behaviors you can do throughout the day and night to help you sleep better. They’ll help you fall asleep faster and reduce nighttime awakenings, improving sleep efficiency.
Here’s what you can do:
The RISE app can tell you the right time to do these sleep hygiene habits based on your circadian rhythm each day.
Losing belly fat isn’t easy, especially as you can’t spot reduce it, so you’ll need to focus on losing fat overall. But there are some things beyond diet and exercise that you may be overlooking when it comes to fat loss.
Keeping your sleep debt low and aligning with your circadian rhythm can stop more belly fat from forming, and help make losing existing body fat easier.
The RISE app makes those two things much easier to do. The app keeps track of sleep debt as you pay it back and predicts your circadian rhythm each day so you can sync up with it. It also tells you the best time to do sleep hygiene habits, which will help you sleep easier and longer each night.
Without exercise, you can lose belly fat overnight by focusing on how much sleep you get and the timing of this sleep and your meals.
You can get a flat stomach by reducing belly fat by meeting your sleep need and syncing your sleep and meals with your circadian rhythm each day.
Weight loss takes time, especially losing fat from the belly. But you can speed up the process by focusing on your sleep and circadian rhythm. Get enough sleep and be sure to eat and sleep in sync with your body clock.
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