Discover How to Lose Body Fat While We Sleep

We often hear the phrase “burn fat while you sleep” tossed around in the world of health and wellness. It sounds too good to be true—but it’s not entirely a myth. In fact, our body is hard at work during the night, performing biological processes that include tapping into fat stores. So what’s really going on while we are tucked under the covers? Let’s break down the science of how and why we use body fat while we sleep.

Even in our deepest sleep, our body is never truly at rest. It continues to perform vital tasks—repairing tissues, consolidating memories, regulating hormones, and, yes, burning calories. This baseline energy expenditure is known as our Basal Metabolic Rate (BMR). It accounts for about 60–75% of the calories we burn each day, and a significant portion of it happens overnight.

To fuel this nocturnal activity, our body draws on various energy sources.

During the early part of the night, our body is still using glycogen—stored glucose in our liver and muscles—as a primary energy source. But as those stores are depleted through hours of fasting, our metabolism starts to rely more heavily on lipolysis, the breakdown of fat cells. This is the foundation of overnight fat metabolism.

Fat stored in our adipose tissue (body fat) is broken down into free fatty acids and glycerol. These molecules are released into the bloodstream and transported to various tissues and organs where they are oxidised—or “burned”—for energy. This process is especially active during REM sleep, when the brain is highly active and requires more fuel.

In fact, our brain needs just as much energy, in the form of calories, at 3 a.m. as it requires at 3 p.m. The brain is slightly different from other organs. Other organs use glucose and fatty acids (small fat molecules) as an energy supply. The cells in the brain have a slightly different cell membrane structure whereby fatty acids cannot pass across this membrane to fuel the brain cells. Once the glucose is fully utilised, the brain instead switches to ketones as its primary energy source. Our liver turns our stored body fat into ketones, which can breach the blood-brain barrier and sustain our brain function whilst we sleep.

As well as brain fuel, the fat our body burns during sleep serves other multiple important purposes. Muscle maintenance is key, and our body uses energy overnight to repair and maintain muscle tissue. By tapping into fat stores, it spares protein breakdown, helping to preserve lean body mass. Maintaining a stable body temperature also requires energy, especially in cooler environments. Fat oxidation helps fuel this thermoregulation. Many restorative processes, like cellular repair and detoxification, require energy—from immune system function to the removal of cellular waste. Fat provides a slow, steady fuel source ideal for these long-haul tasks.

Several key hormones help orchestrate this fat-burning process during sleep:

• Growth Hormone: Secreted in pulses during deep sleep (especially early in the night), growth hormone promotes fat breakdown and helps preserve lean muscle mass. It also supports cellular repair and regeneration.

• Cortisol: Often demonised as a stress hormone, cortisol follows a natural circadian rhythm, peaking in the early morning. This rise helps mobilise energy—often from fat stores—so we can wake up alert and energised.

• Insulin: Lower insulin levels overnight encourage the body to access fat stores. Since insulin inhibits fat breakdown, its night time dip is essential for fat-burning to occur.

So, if we wish to, how can we optimise overnight fat burning? While we can’t control our metabolism minute-by-minute during sleep, certain habits can enhance our body’s natural fat-burning rhythm. It makes sense that we don’t eat too late: if we fuel ourselves before bedtime, we boost our glucose and glycogen stores. This may give us extra fuel while we sleep, taking longer for our bodies to reach a fasting status and thereby delaying body fat burning as a source of energy. Late-night meals also can keep insulin elevated, preventing fat breakdown.

In contrast, if we are looking to gain weight, then we should ensure a bedtime snack to fuel our slumber. This is of importance if there is very little excess body fat, as we do not wish the body to use protein, mainly found in muscles, as its source of energy. This would equate to a deterioration in lean muscle mass, which is never a desirable consequence.

We can also optimise fat burning by prioritising quality sleep. Poor sleep disrupts hormone balance, reducing growth hormone and increasing cortisol in unhealthy ways.

It is also worth noting after waking up we will remain in this fasted state until we break our fast with of course breakfast. During this fasted state, we will continue to use body fat as our energy source—literally burning body fat until breakfast. Though we should not skip meals, delaying the fast break until slightly later in the morning enables us to lose weight. For those of us that may have looked to shorten our eating time window, we can begin to understand how starting eating later in the day and finishing earlier drives weight loss. Should we be looking to gain weight, then of course the opposite is true, and we should eat as early as possible in the day, focusing on a protein-rich breakfast and eating later into the day.

To wrap up, our body is a metabolic powerhouse—even when we’re asleep. By understanding how fat is used during the night, we can better appreciate the value of decreasing our eating time window, good sleep hygiene, and smart daily habits. So the next time someone says we're not doing anything while we sleep, we can confidently tell them otherwise: we’re literally burning fat in our dreams.

Best wishes,

Nina Thomas

BSc (Hons), MSc, ANutr

Registered Associate Nutritionist & Food Scientist

Bridgford & Bingham Nutrition              

21 Brickyard Lane

East Bridgford

Nottingham

NG13 8NJ

Call: 07958 765337

​www.bridgfordbinghamnutrition.co.uk