What Your Body Does During Sleep, According To Science

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We spend almost a third of our lives sleeping. And while it seems like nothing happens to our bodies as we slumber, that couldn’t be further from the truth. Even in the deepest stage of sleep, the human body continues to spend energy on maintaining homeostasis and other vital functions. But where exactly is this energy going, you ask? Well, let’s break it down.

How Many Calories You Can Burn During Sleep

Your body does a lot of work while you’re snoozing. And to complete all of this work, it burns your accumulated reserves from the day — glucose and glycogen, in particular — in order to meet the energy demand.

According to studies and calculations, a 125-pound person burns approximately 38 calories per hour of sleep. If you multiply this amount by seven or eight hours of sleep, which is the average for most people, you burn between 368 and 400 calories per night. That’s the equivalent of going for a 90-minute, moderately paced walk. And if a person has a very active lifestyle and a large amount of muscle mass, they can burn even more calories while they sleep — between 58 and 60 calories per hour, or 464 to 480 calories per night.

Pretty impressive, right?

Your specific calorie burn while you sleep is determined by a little something called your basal metabolic rate (BMR). This calculation is the minimum amount of energy necessary to fuel your body’s vital functions like maintaining a stable body temperature, breathing, keeping your blood circulating, allowing for cellular repairing and growth to occur, and making sure your muscles can contract. Your personal BMR depends on both your daily caloric intake and overall level of physical activity. On average, the BMR for an adult male is 1,700 calories, and women have a BMR of approximately 1,500 calories. The higher your BMR, the more calories you burn while sleeping.

What Happens Inside Your Body During Sleep

During wakefulness, you’re in a metabolic stage called catabolism. This period requires the body to actively use stored energy for its needs. During this stage, your levels of adrenaline and other steroids, as well as stimulating neurotransmitters, are high.

When you fall asleep, your body switches to the anabolic stage and begins to store energy for a new day. At this time, levels of adrenaline, cortisol and other substances responsible for sustained activity decrease. This reduction is reflected in a few important physiological processes:

Your body increases its release of the sleep hormone melatonin, which makes you drowsy and helps you fall asleep. When your body temperature rises again in the morning, this hormone level in your blood decreases, and you wake up.
The concentration of the human growth hormone somatotropin increases in your body. This hormone is responsible for all repair processes and cellular growth.
Estrogens and androgens are also secreted while you sleep.

All of these processes require energy. To provide energy at the proper levels, your body uses glucose and glycogen, which are stored in your skeletal muscles and liver, and breaks down adipose (fat) tissue.

Speaking of adipose tissue, according to a 2014 study published in the science journal Diabetes, sleeping in a colder environment, in addition to encouraging your body to burn more calories, helps you get rid of white belly fat (the kind you want to lose). While brown fat helps regulate metabolism and encourages weight loss, white fat is commonly linked to health issues like type 2 diabetes. And you don’t have to be shivering cold in your sleep — even dropping the thermostat from 75 degrees to between 68 and 70 degrees can create a slight slimming effect.

How Sleep Phases Affect Energy Distribution

Now that you know how much energy is spent while you sleep, let’s dig into where it’s used. Brain activity during sleep is still a mystery for scientists. Yet, they have discovered connections between outbursts of neurological activity during sleep and the various different sleep phases.

In general, your sleep can be divided into two major phases: NREM (non-rapid eye movement) and REM (rapid eye movement). The NREM phase is divided into four stages.

Stage 1: Drowsiness, which usually lasts between 15 and 20 minutes. This is the transition from waking to sleep.
Stage 2: Light sleep, which is when your body temperature begins to drop and your heartbeat slows down. This stage also lasts approximately 20 minutes.
Stage 3: Moderate sleep, which is when any visible muscular movement stops.
Stage 4: Deep sleep, which lasts about 30 minutes.

The REM phase, in turn, has some significant differences from the deep sleep stage of the NREM phase. In fact, your brain activity during REM sleep closely resembles what is observed in wakeful moments. Sometimes, it even surpasses it. So that’s where all of those calories are used.

When bursts of brain activity do occur during REM sleep, they typically come from the main active brain structures: the visual cortex, thalamus and some subcortical nuclei, as well as the hippocampus and amygdala. These areas are responsible for emotional regulation, the formation of memories and the processing of visual images that you observe in your dreams.

These pathways are what make REM sleep so important for your overall mental health and cognitive function (the ability to learn and memorize information, as well as generate new ideas). And that’s why it’s never a bad idea to make high-quality sleep a top priority.

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