What's Your Body Up To During Sleep?

May 13, 2021


What's Your Body Up To During Sleep?

Before the 1950s, most scientists thought human beings were mentally and physically inactive during sleep, but modern research shows this is not the case.

Throughout the night, your brain and body actually do quite a lot of work vital to your mental, physical and emotional health.

So, what’s really going on inside your mind and body during sleep?

For this answer and more critical information on the importance of quality shuteye, read on for our Living Proof Guide to Sleep:

The Four Stages of Sleep

Many people have heard of the “four stages of sleep,” but detailed information on each is less widely known.

Guided by research from Harvard Health, let’s explain the nature and purpose of each stage to help you better understand your sleep cycle:

Stage 1

The first stage of sleep is the briefest and lightest (~5 minutes), and it marks the initial descent into non-REM sleep.

Telltale signs of this stage include:

  • Relaxed Muscles
  • Drop in Body Temperature
  • Reduced Eye Movement 

Electroencephalogram (EEG) readings also show this stage features the low-frequency production of alpha waves initially, followed by even lower-frequency production of theta waves -- signaling increasing relaxation.

Stage 2 

The second stage of sleep is notably longer (~10-25 minutes) and brings about a different mix of physical symptoms and brain activity.

Telltale signs of this stage include:

  • Minimal Eye Movement
  • Slower Heart Rate
  • Slower Breathing

EEG readings of this stage are fascinating, as your brain experiences a flurry of unique activity:

Large and slow brain waves alternate with “sleep spindles,” which are brief periods of sped-up brain waves averaging around half a second in length. 

Among most researchers, the consensus is these latter periods signal the shift of your brain’s focus from outside information to internal memory organization.  

Intriguingly, these readings also show the incidence of brain wave patterns called “K-complexes,” which many scientists believe to be a defense mechanism allowing us to quickly wake up to internal or external stimuli. 

Stage 3

In the third and final stage of non-REM sleep, your brain and body begin to fall into a deeper and more prolonged (~20-40 minutes) phase of sleep.

Telltale signs of this stage include:

  • Slower Breathing
  • Decreased Blood Pressure
  • Dramatically Slower Pulse (20-30% of standard rate)

EEG readings at this point show the presence of large, slow-moving delta waves that accompany a reduced response to stimuli, deeper relaxation and even the incidence of bodily movements (“tossing and turning”).

REM Sleep

The REM stage of sleep is the lengthiest (as long as one hour) and most famous of the bunch, as it initiates the brain activity that leads to dreams.

Telltale signs of this stage include:

  • Rise in Body Temperature
  • Hyperactivity in Sympathetic Nervous System
  • Rapid Eye Movement

EEG readings of REM sleep show that brain wave activity in this phase sharply increases and is very similar to waking hours. It is this dramatic uptick in mental activity that contributes to the formation of dreams.

What Happens in Your Brain and Body

With this essential knowledge of the four sleep stages in tow, it’s time to peel back the curtain on the mystery of what your brain and body do during sleep:


It’s easy to conflate sleep with the “shutting off” of your brain, but this couldn’t be further from reality.

While you move through the five stages of sleep, your brain is highly active in ways you may not have considered.

In the early, lighter stages of sleep, your brain still processes sensory information around you -- albeit at a slower and less effective rate than in its fully awake state.

In these periods, certain stimuli may wake you (such as hearing your name or receiving a tap on the shoulder), but your mind is entering a state of active rest.

As we move to deeper, slow-wave stages of sleep, your brain begins a flurry of activity that’s essential to its strength, agility and growth.

During these incredible sleep stages, your brain can:

  • Categorize and consolidate information from the day, helping you identify patterns and organize your thoughts 
  • Build long term memories
  • Regulate your mood to prepare you for the rigors of the following day
  • Rid itself of harmful forms of toxic waste, which scientists consider a likely cause of neurodegenerative diseases 


While there’s an understandable emphasis on brain activity during sleep, quite a lot happens in your body as well.

While physical reactions vary based on the specific sleep stage, common bodily developments to aid sleep include:

  • Fluctuations in Body Temperature
  • Decreased Breathing and Heart Rates
  • Reduced Kidney Function (minimizes the need for urination)
  • Relaxed Muscles

These basic developments help you fall and stay asleep, but the sleeping body also performs more complex duties essential to your health and development. 

These essential duties include:

  • Release of Growth Hormones 
  • Repair of Damaged Muscles
  • Appetite Regulation
  • Strengthening of Immune System

It is these crucial consequences of sleep, among others, that make it so important.

To be at its best, your body must receive the precious time it needs to repair and strengthen itself.

Risks of Sleep Deprivation

So we know that sleep provides essential benefits to your body and mind, but what are the risks of ignoring some much-needed shuteye? And do they vary based on your sex?

Let’s dive into the primary perils of sleep deprivation for men and women to give you critical information and added motivation to improve your sleep schedule:


Unfortunately for women, science shows they are more prone to the adverse effects of chronic sleep loss than men.

According to a comprehensive study on women and sleep from Duke University, women who battle sleep deprivation face elevated risks of:

  • Heart Disease
  • Type 2 Diabetes
  • Undesired Weight Gain
  • Depression

Though gender-based studies on sleep are still a burgeoning field, many scientists posit that female hormones might be why women are at greater risk for the effects of lost sleep.


While men may be less prone to the negative impacts of sleep deprivation, the effects of inadequate rest still pose serious risks.

According to joint research and testimonials from the American Sleep Apnea Association and Men’s Health Network, sleep-deprived men can face issues such as:

  • Low Testosterone Levels
  • Infertility
  • Erectile Dysfunction
  • Increased Rates of Depression

Finding Your Ideal Sleep 

Now that you know more about the science of sleep, let’s put into perspective what a “good night’s sleep” actually means:

According to research from the National Institute of Health (NIH), the average adult needs 7-8 hours of sleep per night to function at their best.

While this amount can be achieved through nightly sleep or napping, science shows that night-time sleep provides enhanced benefits to an afternoon nap.

Similarly, it’s also less practical to use weekends or other off days to catch up on lost sleep, as it is nearly impossible to pay off your “sleep debt” (amount of sleep hours missed) with irregular added shut-eye.

So, no matter how much you currently struggle to get the recommended 7-8 hours of daily sleep, always aim to improve your sleep schedule on a daily basis rather than trying to make up for lost time now and then.

For any questions about nutrition, self-care or other healthy living protocols, feel free to contact us at any time.