It goes without saying that getting enough sleep should be one of the most important things for anyone looking to optimise their health, productivity and overall well being. On top of that, serious health consequences such as Alzheimer's diseases, diabetes, cardiovascular disease, depression, anxiety and certain cancers can all be linked to poor sleep habits.

Many population studies have clearly demonstrated that on average, people who sleep less than 6 hours per night tend to have a higher body weight, BMI, body fat percentage and abdominal circumference meaning it definitely needs to take priority if you are looking at making a serious change to your physique.

But how much of an impact does it really have on your fat loss results when examining sleep objectively? Granted, the health consequences of not getting enough sleep should encourage anyone reading this to make it a priority but from a pure fat loss perspective, does it actually prevent you from losing body fat?

We know that to drop body fat, we need to create an energy gap by consuming less energy (food) than we are expending (activity). Very simple in theory, not so simple in practice. This won’t be covered in this article but if you did want to learn about how fat loss works, check out this article.

If we now understand that our results are based on the relationship between the calories we consume vs the calories we burn, figuring out how sleep impacts both of these variables is the most logical course of action.

Let’s dive in!

Energy Intake

Being able to manage your hunger and satiety are one of the hallmark characteristics of a successful diet. As you lose more and more weight, the propensity for your hunger levels to increase at a similar rate which is why the last few pounds feel exponentially harder to lose in comparison to the first few pounds. 

The primary hormones responsible for regulating hunger and appetite are leptin and ghrelin. Leptin is secreted by fat cells and acts like ‘the governor’ of the metabolism. When we have a significant amount of leptin in circulation, our brain receives the message that we have adequate energy stores. The result is a decrease in appetite with a simultaneous increase in energy expenditure. A win win! 

Unfortunately, as we lose more body fat our leptin levels also decrease (as we have less fat in our fat cells) while ghrelin tends to increase. Ghrelin is responsible for increasing our appetite meaning when levels are high, we are getting the signal to consume more calories. With higher ghrelin and lower leptin levels, the brain is registering an energy shortage leading to a drop in energy expenditure and an increased desire to eat.

Just to be clear, this definitely doesn’t mean that fat loss comes to a complete stall. Remember, these changes in hormones simply come with the territory when trying to drop body fat. Many have tried to hack the system by including cheat meals or refeeds to temporarily spike leptin to “stoke the metabolism” but outside of giving the dieter a psychological break (which has benefits in some contexts) the increase in metabolism you get doesn’t lead to fat loss. As soon as you start dieting again, leptin levels drop just as quickly.

A reduced metabolism is part and parcel of losing body fat, and that is completely fine but we still want to be doing everything we can to at least slow down the reduction to some degree. This is where sleep can really shine!

Individuals who chronically do not get enough sleep also have a tendency to have a higher BMI along with lower baseline levels of leptin (15% less for habitual sleepers getting 5 hours per night in comparison to 8) and higher levels of ghrelin (15% more for habitual sleepers getting 5 hours per night in comparison to 8). This means that even though your body has more than enough fat stores to “tell” your brain that you have enough energy, your brain doesn’t actually receive that message when you are sleep deprived. 

This study compared one night of sleep restriction (4 vs 8 hours) on 12 healthy young men and observed their ad libitum (no restriction) eating and exercise habits. On average, the increase in calories the following day after 4 hours of sleep was 559 calories along with notable changes in perceived hunger prior to breakfast and dinner. This was enough to put each of them in a positive energy balance as their activity remained virtually unchanged from an expenditure perspective, demonstrating a temporary dysregulation in hunger hormones. 

What does this mean for fat loss? Well technically, you could be at 20% body fat but have the same hunger levels of someone who is already at 10% meaning you are much more hungry than you need to be, you’re expending less energy than you could and your desire for “comfort” foods is also high. 

The combination of all these factors makes it much easier to overconsume, therefore having the potential to impact your fat loss efforts to a great degree. A key point to remember here though is that unless you DO consume the excess calories, the lack of sleep in and of itself isn’t going to prevent you from losing body fat. 

Energy Expenditure

We have previously discussed the four components that make up the “energy out” side of the equation which are:

• BMR (Basal Metabolic Rate)
• TEF (Thermic Effect of Food)
• EAT (Exercise Activity Thermogenesis
• NEAT (Non- Exercise Activity Thermogenesis)

Understanding if and how sleep loss affects these variables can tell us just how much of an impact it will have on your fat loss endeavours.  

Basal Metabolic Rate

During normal sleeping hours, basal metabolic rate appears to be reduced by 15% in comparison to waking hours. The main reason for this is to give the brain a chance to clear out metabolic waste products that have accumulated over the course of the day. This is one of the explanations as to why we find it much easier to sleep in cooler environments vs warmer environments. The cooler our body temperature, the more efficiently we can clear waste products.

From an energy expenditure perspective, there actually doesn’t seem to be any difference in resting metabolic rate between getting an optimal amount of sleep (6-8 hours) vs short sleep (3.5-5 hours). This study had 10 healthy women go three days with their normal sleep schedule (7-8 hours per night) and then three days with less than 4 hours per night and examined how their basal metabolic rate responded. 

Interestingly, no changes were seen whatsoever meaning that a big portion of the energy expenditure pie remains the same whether or not you get adequate sleep. During periods of extreme sleep deprivation (literally no sleep for an entire 24 hour period) there does appear to be a 5% reduction in RMR when measured in the morning meaning if you burn 1500 calories per day on a normal day, this number is reduced to 1425 calories. But to counteract that, your total daily energy expenditure is increased due to simply being awake for the hours you normally would be sleeping, meaning you actually end up expending more energy when you don’t sleep. 

Based on the current data, the impact of sleep loss on BMR is virtually non-existent and total energy expenditure appears to increase. 

Thermic Effect of Food

This aspect of our metabolism relates to the energy that is expended in digesting certain nutrients. Measured thermic effects of nutrients are 0-3% for fat, 5-10% for carbohydrates and 20-30% for proteins and the expenditure itself comes from producing energy, contributing to metabolic processes and storing nutrients within the body.

It usually accounts for ~10-15% of the total energy expenditure for the day depending on the size and quality of the foods eaten. Whole foods usually have a higher thermic effect than processed foods for example.

When we do not get sufficient sleep, our ability to handle glucose (or food in general) can be impaired substantially. In this very famous study, eleven healthy young men were subjected to 4 hours in bed for 6 nights followed by 12 hours for 7 nights to recover from sleep debt. An intravenous glucose tolerance test was performed on the sixth day. Sleep deprivation resulted in reduced glucose tolerance (rate of glucose clearance) by 40%, inducing a temporary state of insulin resistance. 

I covered insulin and insulin resistance in more detail here if you would like to learn more.

Research has shown that individuals with insulin resistance have a lower TEF in comparison to healthy controls. This means if two individuals were both eating 2000 calories per day, the healthy individual may expend 200-300 calories from digesting their food whereas the individual with insulin resistance may only get 100-150 calories. 

How this translates into acute bouts of sleep loss may be very negligible (50-100 calories less than what you normally would burn for the entire day of eating) but still, a poor week of sleep could potentially lead to a reduction of 350-700 calories expended which can be impactful depending on where you are within your fat loss phase. For reference, that would be like missing 2 x 30-40 minute moderate intensity cardio sessions.

I would say that sleep loss can have a minor impact on this aspect of your metabolism but considering TEF only takes up a small percentage of your total energy expenditure, it’s still very small in the grand scheme of things.

Exercise Activity Thermogenesis

Interestingly, your ability to perform cardiovascular exercise at low to moderate intensities (which is what we recommend during fat loss phases) remains virtually unchanged. 

“Sleep deprivation of 30 to 72 hours does not affect cardiovascular and respiratory responses to exercise of varying intensity, or the aerobic and anaerobic performance capability of individuals. Muscle strength and electromechanical responses are also not affected.”

This is not surprising as it’s quite common to hear of incredible performances from military personnel where sleep restriction is enforced as part of their training. Where there is a large difference though is the perceived difficulty of said task along with your motivation to actually exercise due to the fatigue that comes with sleep restriction.

What may feel like a 5/10 on a normal day now may feel like an 8/10 despite producing the same amount of work. As someone who engages in cardiovascular work daily, I personally have felt this myself and even though you can get through the session it’s definitely a lot more enjoyable when you are adequately rested. 

The same can be said for resistance training too but there does appear to be a lot more variability here. Personally, I have had some of my all time best sessions after a night of sub par sleep and have had horrible sessions despite sleeping well. Our feelings can be very deceptive, don’t rely on them to gauge whether or not you should train that day.

The key takeaway from this aspect though is that even if you haven’t had the best night of sleep, don’t skimp on your training. Most of the time, you can actually perform at a much higher level than you think. Also, it teaches you to still show up even if you don’t feel like it, which is one of the key components in staying in shape for life.

Non-Exercise Activity Thermogenesis (NEAT)

The final component contributing to total energy expenditure and the one that has the most variability amongst individuals in general is NEAT. This accounts for all the energy you expend outside of formal exercise such as incidental steps (not including planned walks), fidgeting, postural corrections or anything that you don’t consciously know you’re doing. 

There does seem to be a large genetic component here but even still, most people can increase their NEAT to some degree by purposefully manipulating their environment to make them move more. For example, not owning a car will naturally make you walk more or doing a job that requires you to be on your feet more. 

When we are dieting to lose body fat, NEAT will also reduce as part of the metabolic adaptation process that we discussed earlier. This is in response to our brains detecting an energy shortage meaning it will do what it can to bring you back to homeostasis or equilibrium.  

Unfortunately, there doesn’t seem to be any direct research looking at NEAT and sleep restriction. Every study I came across has examined total energy expenditure for the day without isolating NEAT which in and of itself, is extremely hard to measure anyway. But if we were to extrapolate what we know about NEAT (a large genetic component, environmentally driven and very responsive to perceived energy status) along with how we feel after not sleeping enough (higher levels of fatigue, low mood, poor brain function) it makes complete sense for the body to want to preserve energy, not expend it. 

This can be worked around to some degree by purposefully going out for more walks or engaging in more activity in general. Yes, this does mean that it’s no longer NEAT but if you’re consciously making an effort to move more, it doesn’t matter from a fat loss perspective. 

Practically speaking, this is where having a step count target to hit each day becomes very handy and as long as you make sure to hit it, you’re doing everything you can to continue moving in the right direction. 

Muscle Mass

So far, the impact of sleep loss on overall energy expenditure seems quite inconsequential but it’s important that we look at the bigger picture of metabolism. Remember, maintaining muscle mass is just as important as dropping body fat when it comes to achieving a successful body transformation but having more muscle mass in general goes far beyond the notion of “just looking better”. 

Having low levels of muscle mass is one of the predisposing factors to conditions such as neuromuscular disease, sarcopenia, frailty, obesity and type 2 diabetes. On top of that, population based studies have shown that these conditions are 15-30% more likely to occur in individuals who do not get enough sleep.

The accrual of muscle mass can simply be summarised as protein synthesis being higher than protein degradation. If you are producing more than you’re breaking down, you’re not going to be losing any muscle mass. The keys to triggering this protein synthesis are applying adequate muscular tension on your muscles from resistance training and eating sufficient protein/calories. From here, hormones such as testosterone and IGF-1 promote the growth of new tissue (anabolism) while simultaneously suppressing the genes that are responsible for protein breakdown (catabolism) while on the opposite side of the spectrum, cortisol (a stress hormone) drives the breakdown of tissue while suppressing the anabolic pathways.

Sleep restriction alters our hormonal environment quite significantly by skewing cortisol levels up while bringing testosterone and IGF-1 down meaning our ability to adapt to training can be impacted a lot. 

This 2021 study examined how one night of sleep deprivation impacted muscle protein synthesis rates amongst 13 healthy individuals (male and female) and found that protein synthesis rates were reduced by 18% on average the following day. 

“By studying participants in the postprandial state, we were able to conclude that acute sleep deprivation induces anabolic resistance, decreasing the capacity of the muscle to respond to the typical anabolic stimulation triggered by dietary protein intake. These results have potential, far‐reaching implications for the musculoskeletal and metabolic health of populations including shiftworkers, new parents, students, and older adults, who are at increased risk of acute and/or chronic sleep loss.”

Fundamentally, when you don’t sleep enough, your ability to benefit from your protein rich meals is reduced leading to less muscle growth/retention. When you combine the anabolic resistance of protein along with the less than ideal hormonal environment for protein synthesis, it’s quite easy to see how a lack of sleep can negatively impact your overall body composition even if you can provide the stimulus in the gym. 

Losing More Muscle Than Fat?

In 2010, there was a research paper comparing the fat loss efforts of 10 overweight individuals who were put on a diet that equated to 90% of their BMR (quite low) for two separate time periods. For 14 days the participants were able to sleep an average of 8.5 hours per night and then during the other 14 days, only 5.5 hours. Both time period were separated by several months.

The research was conducted in a sleep labradory and the scientists had 100% control over their diet for the entirety of both experiments.

During both interventions, the participants lost an identical amount of weight (7lbs on average) but what differed was where that weight loss was coming from.

While on 8.5 hours sleep each night, over 50% of the participants’ weight loss consisted of fat. While on 5.5 hours sleep each night, approximately 25% of the participants’ weight loss consisted of fat – in other words, they lost 55% less fat than when they were sleeping 8.5 hours. Also, when sleeping only 5.5 hours per night other measurements such as their resting metabolic rate (RMR), energy expenditure, leptin and ghrelin levels were all negatively impacted.

Even though the paper had its limitations, it does show that all else being equal sleep deprivation can have an impact on the type of weight you do lose. If you were to extrapolate those results over the length of a typical fat loss phase (say 12-24 weeks) you could lose 40lbs on the scale, yet only net a 10lb loss of fat for your efforts. 

It would be interesting to see if the same experiment was done while the participants were engaging in progressive resistance training and eating adequate amounts of protein (the macronutrient information wasn’t provided) but I would guess it would just minimise the gap, not erase it completely. 

The key takeaway here is that if you want the majority of your weight loss to come from body fat, then sleep is definitely going to have an impact. 

Does Sleep Loss Prevent You From Losing Fat?

The short answer? Absolutely not. 

From what we have covered, sleep restriction in and of itself has virtually no impact on your total energy expenditure and as long as you stick to your diet, you will continue losing fat.

The long answer? It can definitely make it harder while also increasing the chances of muscle loss too. 

Inadequate sleep can have profound indirect impacts on your energy intake by manipulating your hunger and satiety hormones, blunting your body’s ability to handle glucose effectively, increasing your desire to snack on high calorie dense foods and not allowing you to get the most out of your protein intake. 

Even though your ability to exercise isn’t hindered, your desire and motivation to want to train along with how hard your sessions feel can all be impacted in a negative way. Also, the many hormones that are responsible for recovery and facilitating muscle growth/retention are downregulated while the more catabolic hormones are upregulated.

All of these factors can lead to an overconsumption of total calories coming from fat and carbohydrates, poor response from training, less muscle protein synthesis and a higher percentage of weight loss (if any) coming from lean body mass.

Handling A Bad Night’s Sleep

It’s important to remember that even if you do have a bad night's sleep (which happens to everyone, including one of the best sleep scientists in the world Matthew Walker) you can still make the day a productive one to keep you moving forward. 

Knowledge is power and now that you know that it’s completely normal for your brain to want more food the following day, you can put strategies in place to make sure you stick to your nutritional strategy. For example, just accepting that you’re going to be a little more hungry and that it’s “fake hunger” can be all you need to stay the course or you can include some more diet sodas in between meals as our propensity to want more sweetness is higher when sleep deprived. 

Make sure that you go out and get your steps in, you take a bit of extra caffeine pre workout to perform at your best while taking more time to warm up to your harder sets. Yes, it may feel harder initially but once you get going you will feel 10 fold better.

Key Takeaways

If you truly want to get the most out of your fat loss process, optimising your sleep to the best of your ability is going to be a very smart move but it’s not the end of the world if you have a few nights of poor sleep. 

There is still a lot you can do and in my opinion, how you operate under a bit of sleep deprivation can be a great way to test the strength of your structure, strategies and systems. It’s not uncommon for many to experience sleep disturbances towards the end of their fat loss phase (another fun trait of metabolic adaptation) meaning there are going to be days where you just need to push through.

Is it ideal? Definitely not but if you can continue nailing everything under those conditions, it gives you the confidence to know that if a similar situation arises in the future you will be able to handle it like a boss.

If however, you are sick of not getting enough sleep try the following tips and see if you notice an improvement. We didn’t include any supplements as when done effectively, lifestyle and behaviour can go a very long way.

• Install F.Lux on your laptop if you are prone to using electronics late into the night. Ideally no electronics would be the best but we need to be realistic too.
• Try not to eat too close to bedtime, especially if it is a heavy meal (at least 4 hours prior).
• Have a wind down routine you do every day as this will start to prepare the body and mind to sleep. Our bodies love routine.
• Keep a similar sleep/wake time, even on weekends.
• Use a form of continuous, non-distracting ambient noise (e.g. a fan or “white noise” machine / phone app).
• Maintain a cool temperature and adequate ventilation in the bedroom.
• Avoid napping during the day (if you must, limit the nap to less than 30–45 minutes).
• Reserve the bed for sleep and sex only. Do not use the bed for work, watching television, or using other electronics.