Why A Lack Of Sleep Is Making You Fat

I have written a lot about fat loss, and a lot about sleep in my blog articles. But I have never linked the two in great detail. After responding to one of the countless emails I get each day asking for fat loss advice, I noticed the reader wasn't too keen on my suggestions - sleep more, get some sun, de-stress etc. etc.

They simply wanted to know how many calories to eat, when to exercise and what fat burning supplements to take. I sent backlinks to my articles on the relationship between hormones and fat loss, stress and fat loss, and why tracking calories is a waste of time. I have covered all these topics in great depth.

But then I realised I have never explained the link between sleep and fat loss in great detail. So this article is for every reader who is seeking to lose weight. Hopefully, the points below will help showcase to you the importance of quality sleep when it comes to fat loss, and why a good sleep in is better for your waist line that a 5am spin class!


Part 1: The Facts -  Why Lack of Sleep Is Making You Fat

We have been misled that the key to fat loss is simply to eat less and move more. That's all there is too it right? Wrong. I have explored this topic in great detail in my article 'The Problem With Counting Calories For Fat Loss', but in essence, there is more to fat loss than what you put into your mouth.

Fat loss (or fat gain) is often a hormonal issue. And one of the biggest drivers of hormones in our body is our sleep. Hence the title of this blog - lack of sleep makes you fat.

Don't believe me? Let's look at some peer reviewed research papers:

  • A study using data from the National Health and NutritionExamination Survey I (NHANES I) found that people got less than seven hours of sleep a night were more likely to be obese. With people who slept less than 5 hours a night were nearly twice as likely to become obese than those who slept 7- 9 hours (1).
  • In the 2012 review titled 'Does inadequate sleep play a role in vulnerability to obesity?' the author looked at the relationship between sleep duration and BMI and obesity rates. A clear link was established between lack of sleep and higher body weights. (2)
  • An earlier review titled 'Short sleep duration and weight gain: a systematic review' concluded the same thing - short sleep duration led to increased weight (3)
  • In 1986, 68,000 women had their sleep habits tracker. After 16 years this data was analysed. The findings concluded 'short sleep duration is associated with a modest increase in future weight gain and incident obesity.' (4).
  • A large 2008 meta-analysis (looking at various published studies on the subject) supports all the above findings concluding 'studies from around the world show a consistent increased risk of obesity amongst short sleepers in children and adults.' (5)

Worst, the link between lack of sleep and fat gain is also well established in children and teens:

  • A 2014 paper published in the Journal of Paediatrics remarked that young children who had sleep restrictions (i.e waking to an alarm prior to their natural wake time) had higher overall and central adiposity in mid-childhood. (6)
  • A 2018 review concluded : "the relationship between shortened sleep and increased risk for obesity has research support internationally" (7)
  • And a recent paper published in the Journal of Sleep set out to assess the relationship between sleep and obesity in children. After reviewing 42 published papers the authors concluded simply 'Short sleep duration is a risk factor or marker of the development of obesity in infants, children, and adolescents.' (8)

The evidence is clear, lack of sleep is causing our children and us adults to become overweight and obese.  

Why is this the case? How can 6 hours of sleep cause someone to become fatter than someone sleeping 8 hours? 

What is the reasoning behind this? In part 2 below I take a look at the mechanisms behind how lack of sleep is causing us to become fat.


Part 2: The Mechanisms - How Lack of Sleep Is Making You Fat

As I mentioned in the the intro to this article (and have covered in great detail in our articles on this website) fat gain (or loss) is often a result of a hormonal change or hormonal imbalance.

It's why when bodybuilders start taking steroid hormones their body composition changes pretty much overnight. It's why hypothyroid suffers start losing weight when they take thyroid hormone. It's also why a female will start adding fat stores to her body when she falls pregnant. 

Hormones are truly powerful things, and you want them on your side if you are seeking fat loss. Knowing this, let's look at how sleep (or lack thereof) impacts hormones and is making you fat.


Leptin & Ghrelin

I talk about the hormones leptin and ghrelin in my blog article - Why You Need To Learn About Leptin If You Are Overweight. I highly recommend reading this article if you do have problems with weight loss.

As I explain in that article, leptin is regarded as the 'master hormone' when it comes to appetite and body fat levels. Increased levels of leptin suppress appetite. 

Meanwhile, ghrelin has the opposite effect - increased levels of ghrelin increase your appetite (I associate the world ghrelin with a growling stomach). 

The interplay between these hormones plays a vital rule in determining your body composition. In a healthy individual, without any hormonal imbalances, these hormones will maintain an equilibrium between hunger, and body fat stores.

However, it's possible to break this balance - leading to something known as 'leptin resistance'. In the article I mentioned above I take a deep dive into the causes behind this leptin resistance (and how to fix them). One particular issue that can trigger leptin resistance is the lack of sleep. Another is circadian rhythm misalignment (not sleeping as per your environments light and temperature cycles).

Science has proved this to be the case. In the 2004 paper titled Short Sleep Duration Is Associated with Reduced Leptin, Elevated Ghrelin, and Increased Body Mass Index, researchers found that when are sleep deprived, our bodies produce more ghrelin and a decrease in leptin (9).

Remember, ghrelin causes an increase in appetite. 

The researchers went on to remark that lack of sleep is stimulating appetite through the increased ghrelin and decreased leptin levels. in the body.

Lack of sleep makes you eat more.

Scientists no that leptin increases with more sleep, and ghrelin falls with more sleep - helping the body regulate body fat levels. And we now know that lack of sleep has the opposite effect on these hormones. 

In fact, sleeping for 4 hours in two nights increased ghrelin by 28% and decreased leptin by 18%!  (16)

That is a dramatic change if you are trying to fight your will power and lose some weight.

So next time you set your alarm for 5am to hit the 5:30am spin class, think about leptin and ghrelin, you will be better off skipping the class and getting and extra hour or two of sleep! 



Insulin is another hormone that plays a vital role in body fat levels, and is also heavily impacted by sleep (or lack of sleep!)

Insulin's role in the body is to clear sugar from the blood - moving it to where it's needed (i.e. muscles) or where it can be stored for later use (i.e. converted into body fat). You will be familiar with the term insulin resistance - this is a state where insulin cannot do its job, causing all sorts of medical issues including fat gain.

Now here is the scary part, studies have shown that lack of sleep can also impair insulins ability to work. Effectively lack of sleep can put us into a diabetic state (10).

Worst though is the compound effect the hormonal mess lack of sleep causes. We know that lack of sleep creates an increase in our appetite, and now we know that lack of sleep brings about a state of insulin resistance, so guess what happens when you wake up short of zzz's? You are extra hungry and the body cannot clear the sugars you are eating as insulin has been de-sensitised.

If you were looking to create the perfect hormonal state to make someone fat, this would be it. And millions of people are doing this to themselves by simply not getting enough sleep.

For those of you who often sacrifice sleep to hit a project deadline, this finding may scare you - after a week of sleep deprivation, insulin sensitivity decreased by 50%. You are effectively making yourself diabetic, irrespective of that keto diet, or all the training sessions you are doing (11). 

Oh and before I move on - if you were thinking 'well I never go two nights in a row with lack of sleep, this doesn't apply to me', research has shown that even one night of short sleep can create a state of insulin resistance (11). 

Stress & Cortisol

Another hormone that sleep impacts is cortisol. Cortisol is often regarded as our 'stress' hormone. This is true, but cortisol doesn't deserve the evil image that many portray it as.

I talk about the relationship between stress and weight gain in my article 'Why You Need To Stress Less To Lose Fat'. If you think stress is limiting your weight loss goals I recommend you go and read that.

Now cortisol (ideally) follows a natural cycle - it's higher in the morning when we wake up, and decreases in the evening helping us wind down and fall asleep.

If this natural rhythm is broken (i.e. elevated cortisol levels in the evening), not only does it cause falling asleep to be rather difficult (if you have ever experienced that 'wired by tired' feeling where your body is exhausted but your mind is still alert despite it being well past your bed time - then you will know what this state is like!), but elevated evening cortisol levels from chronic sleep loss have been linked to insulin resistance, and obesity. 

Note: Late night exercise and blue light exposure late at night can both cause elevated cortisol levels. For more on this subject I recommend reading -  How Technology and Blue Light Are Ruining Sleep and Making You Sick, Fat and Tired (And How to Fix It)

But elevated evening cortisol and problems falling asleep aren't the only issues. Lack of sleep in of itself is a stressor on the body. Sleep is a time for the body to destress, and deload all of the days stressors (recovery and repair from exercise, elimination of ingested toxins etc).

Not getting adequate sleep not only hampers this recovery/de-stressing process, but actually increases the level of stress on the body (meaning the body needs more sleep in the future to cope with the increased stress load).

Think of it this way, let's assume your stress load is measured by your bank balance (and for many people this is true!) The lower the balance the more stress the body has.

Now we start the day and have 3 choices. Option 1 see's us heading to work and earning $100. At work all our meals are provided. We come home with an extra $100 in the bank account.

Option 2 see's us skip work for the day, and eating the leftovers we have in the fridge for lunch. We end the day at the same balance as we started it. 

Option 3 see's us skip work and head out with our mates for a fancy lunch. We pay for the lunch on credit. We end the day with $50 less in our bank account.

If we convert dollars to stress, option 3 is the most stressful and requires future 'work' (or repair) to get ourselves out of the hole we are in.

Option 3 is also what happens when we don't get enough sleep - not only are we missing out on the goodness we get from sleep (i.e. the money we earn from working), but we also increase our stress load on the body (we burn through cash/energy - which must be repaid in the future).

Pretty much, if you are prone to inadequate sleep, you should appreciate that the body is already on behind when it comes to stress balance, and that going to the gym to do a stressful workout is not going to help (even though you may feel like you benefit from it). Instead, take it easy - eat organic, rest, relax and be sure to get an early night!

Ok, let's bring this all back to fat gain and sleep. We now know that lack of sleep increases cortisol. But what does this mean for our waist line?

Increased cortisol levels are impacting glucose metabolism (the brain uses less glucose when sleep deprived, this may be the reason for that brain fog sensation when you haven't slept enough), decreasing powerful fat burning hormones like testosterone, can lead to insulin resistance, and suppresses melatonin (which I'll cover next).

Oh and one last fact before we look at melatonin. I don't mean to scare you, but sometimes the truth hurts - even one single night of inadequate sleep can double cortisol levels (12). 

Sleep up, it's good for you!



Those of you have have read my article on blue light will be well versed on this topic.

In that article, I explore how artificial light decreases melatonin (and increases cortisol, eek) and this, in turn, makes us sick, fat and tired.

Let me summarise the key points from that article below.

Melatonin is often regarded as the 'sleep hormone' as it's released when we sleep at night (though I think the term 'darkness hormone' is more suited, as it's lack of light that triggers melatonin release).

This melatonin informs the body whether it's day time or night, and this the body can adapt it's metabolic processes accordingly. 

Melatonin also has anti-oxidant properties - helping us ward off disease and fight inflammation.

But in the context of this blog, the most important role melatonin has in the body is related to the impact it has on insulin sensitivity, thermo-regulation, energy expenditure regulation, and leptin and ghrelin levels.

You see, a good night's sleep, a sleep that is aligned to your environments light/dark cycles, allows the body to maintain healthy insulin sensitivity levels, it helps the body 'burn off' excess energy consumed throughout the day through the activation of brown fat tissue and it ensures that optimal levels of leptin and ghrelin are released.

It should be clear how a disrupted sleep cycle and inadequate sleep (in turn not allowing sufficient melatonin to be released into the body) is going to cause a hormonal wreck in the morning - a wreck that is prone to fat gain.

Melatonin's primary role when it comes to fat gain/loss is how it works with leptin.

Let's take a quick look at how melatonin, leptin, sleep and fat loss are interconnected. In a healthy body, the following happens at night:

  1. After 4 hours of sleep in darkness (you shouldn’t be able to see your hand in front of your face when sleeping), melatonin is produced.
  2. When this happens, leptin creates a surge of fat reserves to be released for energy.
  3. Your thyroid is up-regulated, boosting your metabolism (burning off excess energy consumed during the day - this is why I can eat 6500 calories a day and still have abs, because I sleep so much! And wear blue light blockers prior to going to bed)
  4. At the same time, your mitochondria start burning this fuel to maintain a stable body temperature throughout the night.
  5. All this fat mobilisation and burning happens effectively when you are sleeping a solid 7-9 hours a night in line with your environments light cycles.
  6. Elevated insulin (from big meal too close to bed) can shut down this process, along with exposure to artificial light at night - particularly blue light.


Adequate sleep combined with blocking blue light and sleeping as per your natural environment help you stay lean. 

But it's not just sleep duration that plays a role in determining whether melatonin will help you stay trim, a properly aligned circadian rhythm is also crucial. Misaligned circadian rhythms (i.e from night shift work, or jet travel, or simply staying up late exposed to blue light) is linked to increased obesity rates.

This is why I have written about the harmful effects of long haul jet travel in my article 12 Reasons Why Travelling Is Disruptive To Our Health (And How To Remedy It) in my mind it is a recipe for poor health and fat gain.

Growth Hormone

Another hormone that lack of sleep impacts is growth hormone.

Growth hormone (GH) is released in it's highest amounts when we are in slow wave sleep (tip - use the Oura Ring - read my Oura Ring Review HERE - to measure the amount of slow wave sleep you get each night). 

A lack of slow wave sleep results in lowered GH levels in the body, in turn a lack of GH leads to lowered muscle mass and bone density and increases body fat levels. 

This means in regards to fat loss and sleep, GH is both a quantity (duration) and a quality issue. Even if you are getting your 8 hours of sleep each night, if you're not getting adequate slow wave sleep, you're missing out on all the fat burning properties that growth hormone brings to the body.

For ways to increase your deep sleep - check out my article How To Increase Deep Sleep.



Research has shown that skipping sleep can drastically lower testosterone in guys (13). After 1 week of less than ideal sleep duration testosterone levels decreased by a substantial 10-15%.

Testosterone plays an important role in maintaining (or changing) body fat levels. Plus adequate testosterone levels provide more 'get up and go', allowing you to hit the gym hard, and recover from the days stressors.

Sleep Apnea

It is clear that lack of sleep contributes to fat gain. One of the most common issues behind lack of sleep (other than simply not going to bed early enough) is sleep apnea.

Sleep apnea is an obstruction of your airways that lead to disrupted sleep. The bad news is that sleep apnea is most pronounced in overweight people. This creates a negative feedback loop - lack of sleep leads to weight gain, weight gain leads to sleep apnea, sleep apnea leads to lack of sleep which leads more more weight gain...

If you do suffer from sleep apnea, or you are over weight and wake up still feeling exhausted, speak to your doctor about being tested for sleep apnea.

It could turn your life around (and help you drop that weight!)

Impaired Decision Making

Any one who has suffered from a night of bad sleep know's how sort one can be in the morning. You're more like to make decisions that are out of your usual character, more prone to getting into arguments, and more likely to eat the bagel and muffin on offering in the office. 

Increased ghrelin could be the contributing factor to the extra serving of snacks, but the brain is in a weakened state when you are deprived of sleep. This state can lead you to doing things you wouldn't typically do (or say!)

Chronic lack of sleep shuts down the 'higher level' side of the brain. Things like motivation and long term planning are know longer prioritised, as the body works more on a survival state - a state increasing the likelihood of eating junk food you would usually avoid.  

Another reason to this is the bodies lower levels of neurotransmitters when you are in a sleep deprived state. Lower levels of the feel good neurotransmitter dopamine will leave you seeking for a quick dopamine boost - i.e. that deep fried donut on display at the coffee shop!


Lack of Desire to Exercise

Some experts claim that the lack of sleep causes fat loss due to lack of desire to exercise… I don’t buy into this, and I’m sure anyone that has read my article on calories and know’s how important hormones are for fat loss would agree with me.


Seasonal Impact

In the book 'Lights Out' Author TS. Wiley explains how the body changes it's body fat levels with the seasons. This is also seen in animals living in their natural habitat.

The idea goes that animals store fat in times of abundance (i.e. summer). In these seasons the days are long and the nights are short. The light sends a signal to the body saying 'summers here, eat up and get fat for the upcoming winter'.

I don't have any science to back up these claims, but it is an interesting idea.

The link between lack of sleep and fat gain goes that because we aren't sleeping as long as we should, and because we are exposed to artificial light when we aren't sleeping (something that has only been present in recent generations) then the body senses we're living in a season of abundance, thus, eat up and get fat.

The reality is we do live in a time of abundance. We are fortunate that we have access to food every hour of the day, every day of the week.

The problem is there is no upcoming winter, and the days are perpetually long (and warm) even if we live at a northern latitude were the sun only shines for 2 hours a day and the temperatures are constantly below freezing.


Personally, I think this may be an over simplified reason for why lack of sleep leads to fat gain - we know that blue light exposure increases melatonin,  and the impact this has on leptin etc. These are no doubt the mechanism causing the weight gain from inadequate sleep. 

But this idea is worthy of consideration, if you are one who believes that humans (and animals) do best in their natural environment, then it is something that may resonate with you. 

For those that prefer their explanations backed with hard science, then I'm sure the points I have covered above will satisfy you!

Part 3: The Solution - How To Drop Body Fat Levels By Increasing Your Sleep

We now know that:

  1. lack of sleep causes fat gain, and
  2. the mechanisms that cause this fat gain from lack of sleep.

So now to answer two pressing questions:

  • How much sleep is needed? and,
  • How can I improve my sleep?

Let's start with the first question.

How Much Sleep Do I Need?

I'm going to answer this without any science. It's an answer built from years of experience working with 100's of clients, and having monitored my own sleep (and health) for 1000's of nights...

So here goes - It depends!

It depends on so many factors - age, activity, quality of your sleep etc etc etc.

But I know many won't like this answer, so here is a better answer for you - your body know's best. Wake up naturally without an alarm, get out of bed when you feel rested.

I know this may seem impractical for many, but I believe that this is the best way to determine your ideal sleep length. 

If you are waking up to a screeching alarm and feel absolutely groggy every morning, even if you were in bed for 8 hours, chances are you need more sleep.

I personally don't use an alarm clock, and my sleep length varies between 7.5 hours to 9 hours. 

Actually, I can determine my exact average sleep duration as I have been using my Oura Ring sleep tracker every night for the past few years (read my review HERE). I crunched the numbers and here is what I found:

  • My average time in bed is 9 hours 21 minutes
  • My average sleep time is 8 hours 17 minutes
  • My average deep sleep time is 1 hour 26 minutes

My total sleep duration overlaid with my total bed time - thanks to my Oura Ring

So there we go. If you want some numbers to target, and if you want to be like me - aim for 8-8.5 hours of sleep!

But enough of the anecdotal evidence and my beliefs, what does the literature say about optimal sleep lengths?

A 2011 study found that those who slept between 6 and 8 hours had the lowest risk of obesity (14). But that studies main focus wasn't optimal sleep duration.

Though a 2015 paper had that as their main goal - to determine how much sleep an adult human needs.

Their consensus statement reads (15):

Adults should sleep 7 or more hours per night on a regular basis to promote optimal health.

  • Sleeping less than 7 hours per night on a regular basis is associated with adverse health outcomes, including weight gain and obesity, diabetes, hypertension, heart disease and stroke, depression, and increased risk of death. Sleeping less than 7 hours per night is also associated with impaired immune function, increased pain, impaired performance, increased errors, and greater risk of accidents.
  • Sleeping more than 9 hours per night on a regular basis may be appropriate for young adults, individuals recovering from sleep debt, and individuals with illnesses. For others, it is uncertain whether sleeping more than 9 hours per night is associated with health risk.
  • People concerned they are sleeping too little or too much should consult their healthcare provider.

It looks like I'm right in the sweet spot!


How Can I Increase My Sleep?

Now we know why we need to sleep more, and how much sleep we need, so now we simply have to get more sleep!

But for some people, this is easier said than done.

Fortunately, I have an enormous amount of resources on this site that will help you sleep more (and lose weight). Here are a few resources to check out:

Also - Check out my book - The Ultimate Guide to Deeper Sleep: 65 Research Backed Tips to Effectively Boost Your Deep Sleep

This solution will cost you a few dollars - but not much more than a price of a coffee (and I can assure you that the content in this book will allow you to give up your coffee habit!)

In my book The Ultimate Guide to Deeper Sleep, Bart Wolbers and I cover 65 scientifically proven ways you can increase your deep sleep quality.

This book is packed full of amazing takeaway tips, with simple action points for you to incorporate into your life.

You can learn more about this book HERE

But if there is one tip I can give you to help it is this - prioritise your sleep!

This may mean giving up on TV, getting to bed earlier, saying no to late night movies or dinners. It means making sacrifices. 

But as you now know, these sacrifices will be worth it!


That's A Wrap!

Thanks so much for taking the time to read my article. If you enjoyed it please say so below, if you think this would be valuable to someone you know please share it with them.

And if you enjoy this sort of material, be sure to check out some of my other blogs and sign up for my email newsletter - I only send out the best material!

Oh, and one last thing - if you do suffer from sleep deprivation, be sure to read my article Bad Night's Sleep? 10 Effective Tips To Overcome Sleep Deprivation


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This blog post was written by Alex Fergus. Alex is a ISSN Sports Nutrition Specialist, Fitness Professional and certified Superhuman Coach who continues to expand his knowledge base and help people across the world with their health and wellness. Alex is recognized as the National Record Holder in Powerlifting and Indoor Rowing and has earned the title of the Australian National Natural Bodybuilding Champion. Having worked as a health coach and personal trainer for over a decade, Alex now researches all things health and wellness and shares his findings on this blog. Learn more about Alex's Credentials HERE.

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