We go to bed each night and we wake up in the morning. But what exactly happens to us during the time when we sleep?

Most of us remember those first few moments as our body relaxes and we doze off and then the next thing we really remember is waking up to perhaps some remnants of a dream. What is interesting therefore to know, is what happens to our bodies during that in between time. Do our bodies still use energy? Is our brain resting and turned off? And what role do our dreams play in all of this?

Thus in this part 2 of our Sleep Aware Series, we uncover some of the pieces of what really happens to us when we sleep. If you missed part 1, be sure to check that out too, as we talked about “Why do we need to sleep?

Body Changes During Sleep

To begin with then, let us look at the components of our bodies and what happens to them when we sleep.

1. Energy

While you sleep you are still burning calories and thus using energy. Many people think that we do not burn energy when we sleep, but this is of course a very incorrect assumption. We need to burn energy to fuel our brains still and keep all of our organs, tissues and cells alive.

In fact we can burn anywhere from roughly 400 to 900 calories sleeping for 8 hours (the exact number depends on your weight). Pretty good for doing something so easy and pleasurable, no? The only time that our body does not use ANY energy is when we die.

2. Brain

Another common misconception is that when we sleep our brains are turned off. Well this also is not true. Our brains are still very much at work when we sleep, after all who would be directing your lungs to breathe or your heart to beat, never mind directing numerous other tasks. While there is a “brain resting” stage, there is also a “brain activity” stage, where the brain is just as active as if you were awake.

The difference is that when we sleep our brain is firing messages through neurons at different frequencies and through sometimes different pathways.

3. Organs

We have to understand that when we sleep our organs do not turn off. Our heart is still beating, our brain is still working, our lungs are still exchanging air, our liver is still detoxifying and the list continues.

The difference when we sleep, is that this is usually the time when our organs can look after “themselves” if we may call it that, instead of looking after “you”. May seem like one and the same, but the difference is that during the night, our organs go through various healing and regenerative processes, that cannot always happen during the day.

Hence it is never a wise idea to eat shortly before going to bed because then you are making your body work on the “day tasks” still, when it wants to do “night tasks”. What ends up happening is that both tasks don’t get done properly, and we usually have a hard time falling asleep or staying asleep when we eat before bed, especially large and heavy meals.

4. Hormones

Hormones are little chemical messengers and the activity of each hormone depends on what hormone it is. Many hormones are secreted into the blood during sleep, while some other hormone secretions may be minimized during sleep.

For example, melatonin hormone is produced at dusk and is at its highest when we sleep. It is related to fighting free radicals, and thus also repairs our cells and DNA. Growth hormone is secreted during sleep and is related in part to repair processes of our cells and tissues during sleep.

Follicle stimulating hormone and luteinizing hormone (both in women), which are involved in the maturational and reproductive processes, are also among the hormones released during sleep. In fact, the sleep-dependent release of luteinizing hormone is thought to be the event that initiates puberty and regular release of these hormones can also improve fertility.

Other hormones, such as thyroid-stimulating hormone, are released prior to sleep. While serotonin, adrenalin and cortisol, are released near the morning and work together simultaneously to wake us up. As evening falls, serotonin starts getting converted to melatonin and the whole cycle starts again.

5. Consciousness & Dreaming

In terms of our conscious states, we seem to lose those quickly after we fall asleep and from then on our body operates out of the subconscious level. This is the main reason why we remember so little if anything from our sleep time.

Many of us also have dreams when we sleep and to this day, scientists cannot seem to agree on what exactly dreams are or what role they play. It is however theorized that we all have dreams, but some of us are able to recall them and some cannot. Many theories exist, see Dream Theories from About.com or from HowStuffWorks.com for more information.

6. Metabolic Functions

In terms of breathing, metabolism, heart rate and body temperature, generally speaking they all decrease as we sleep. This is another reason why eating before bed or even having a heavy meal a couple hours before going to sleep can easily lead to weight gain, as your metabolism is most inefficient at this time.

The exception to the rule of decrease, is that our heart rate and even breathing actually significantly speeds up at certain times of the night, depending on what state of sleep we are in.

The 5 Stages of Sleep

Another way of looking at what happens to us when we sleep is through the 5 stages of sleep. Thanks to numerous tests that have been run on both human and animal subjects, by tracking our brain activity through the use of electrodes, we can decipher what neurological activity our brain is going through as we sleep. By studying the amplitude and frequency of our brain waves, we can learn more about what our brain is doing and how this impacts our body.

STAGE ONE – (Non-REM): This is known as Light Sleep and is part of non-Rapid Eye Movement (non-REM) sleep.

This is when we are half awake and half asleep and can be awakened easily at this stage. Muscle activity slows down and slight muscle twitching may occur.

STAGE TWO – (Non-REM): Within 10 minutes of light sleep, we enter True Sleep.

This is the period we spend most of our night in.

STAGE THREE – (Non-REM): Is known as Deep Sleep. This is the period during which our body does its restorative and healing functions.

STAGE FOUR – (Non-REM): Is very close to Stage 3 and is also called Deep Sleep, as well as our restorative sleep. The only difference between stage 3 and 4 is a slight difference in brain waves that is detected. As our body passes through these two stages, it gets ready for normal function the next day.

In both stages, but especially this one, it is extremely hard to be awakened by external stimuli.

STAGE FIVE – (REM): Rapid Eye Movement Sleep is when the brain is at its most active and when dreams occur.

Although the muscles that move our bodies go limp at this stage, other important muscles continue to function. These include the heart, diaphragm, eye muscles, and smooth muscles such as those of the intestines and blood vessels.

REM sleep usually begins about 70 – 110 minutes after falling asleep. We have around three to six REM episodes a night. After REM sleep, the whole cycle begins again.

The deepest stages of Non-REM sleep occur at the first part of the night. As the night progresses, stages of Non-REM sleep get shorter as stages of REM sleep get longer.


Although there are many more details which we can talk about in terms of what happens to us when we sleep, the above tutorial is meant to provide quick, easy and user friendly information for most of us to quickly comprehend.

Ultimately as interesting as it is to know what actually happens to us when we sleep, the most important thing to remember is to get proper sleep in the first place – meaning as regular and consistent as possible with enough hours to wake you up feeling refreshed and rejuvenated.

Join me next in part 3, where we explore the timeless question of how much sleep do we really need.

Till then sleep well!

Other Helpful Sites on The Same Topic

For more help and information on sleep check out the following sites:

  1. What Happens When We Sleep – from Science Daily

  2. What Happens When We Sleep – from Canada.com

  3. What Happens When We Sleep – from Discovery Health

  4. What Happens When You Sleep – from the National Sleep Foundation

  5. What Happens While We Sleep (A Spiritual Perspective)