Where do we go when we fall asleep?

March 14th, 2023

Written by: Hannah Deutsch

We cannot account for about ⅓ of our lives. We spend that time sleeping. During sleep, we are unconscious to the world around us and unable to defend ourselves from threats. Yet, it must be important: we and most animals on our planet sleep in spite of threats to survival. Both sleep’s importance and the uncertainty surrounding what happens during sleep has resulted in sleep being a fertile ground for scientific research. 

In this article, let’s start to unpack the mystery: we will dive into the science behind what exactly is happening in our brains as we sleep. 

Learning and Memory

Our brains store and process information as we sleep. We cannot actively acquire new knowledge during sleep, the two main types (NREM and REM) of sleep play different roles in how our brains learn. NREM, or deep sleep, aids in the storage of information1. REM (rapid eye movement) sleep, the period of sleep where our brains are most active and appear wake-like, aids in the processing of information2. The information processing that occurs during REM sleep is crucial to making connections, which aids in creativity and problem-solving. REM sleep also helps with working through emotional memories, helping us better regulate our emotions while we’re awake. While we may see sleep as a passive state, our brains are doing a lot of work behind the scenes to remember and think critically about the information we’ve recently encountered. 

Besides helping our brains solidify memories so that we don’t forget important things from our day, sleep also prepares us to learn and think critically in the future. When we pull an all-nighter or don’t get enough sleep we are familiar with the results: we have a harder time concentrating, regulating emotions, and making decisions. Sleep-deprivation makes it harder to remember what we’ve already learned and learn new things3.      

Waste Clearance

When we sleep our brains use our cognitive downtime to “take out the trash.” During wake periods, waste accumulates in the brain. Unlike in the rest of the body, where we sweat, exhale, urinate, and poop to whisk waste away at all times of the day, the brain is separate from our body and cannot remove unnecessary proteins and byproducts as easily. Instead, a unique process of waste removal occurs while we sleep. The system responsible for brain waste clearance is known as the glymphatic system4,5

The glymphatic system removes the trash through fluid: our brains are bathed in waves of fluid during sleep, in a process similar to ocean  waves washing over you. For this rhythmic bath to happen, certain conditions have to be met, which happens during sleep. During wake, our brains fire in many different patterns. However, during sleep, our brains have periods of firing rhythmically in waves. The brain requires a regular oxygen supply to function, and during the rhythmic firing, there is a matching blood flow wave (pictured in red, Video 1). When the blood leaves the brain, there is now space for cerebrospinal fluid (pictured in blue, Video 1) to come in and wash away the waste. This cyclical process results in waves of fluid that bathes and cleans  our brains as we sleep.    

Video 1. Video of rhythmic blood (red) and cerebrospinal fluid (blue) flow in the human sleeping brain. 


During sleep, our brains perform different tasks than while we are awake. One key difference is in its metabolic processes. Metabolism is the making and breaking down of molecules in our bodies, including the process of converting food into the building blocks our bodies need. Scientists know different metabolic processes occur in the awake brain vs. the sleeping brain. For instance, during wake, the brain relies heavily on sugar as fuel, while during sleep, the brain relies on fats. Additionally, there is an increase in biosynthetic (the process of making molecules) activity during sleep, which serves to build the molecules necessary for neurons to function properly during the day6. During wake, the brain uses molecules to do its job, and at night, it replenishes its stores, like restocking a shelf. In addition, metabolism during sleep plays an important role in learning and memory: metabolic processes occurring during sleep are required for the storage and processing of memories. Understanding the link between sleep and metabolism is a very active area of research, with a focus on aging, diseases, and understanding why we sleep.  

Why do we sleep?

We know a lot about what happens in the human brain as we sleep including learning, waste clearance, and metabolic processes. However, just because we know much about what is happening during sleep, that doesn’t mean we know why we sleep. Almost all animals sleep: from the most simple to the most complex, and scientists are still searching for a straightforward, unifying explanation for sleep. Because a lot happens during sleep, the most likely answer is that it’s complicated and there are a variety of reasons instead of just one simple explanation. However, that doesn’t seem to satisfy many scientists. If you want to read more about the epic battle between scientists debating why we sleep, I’d recommend looking here, here, and here. We still don’t know where we go when we fall asleep, but scientists are trying to find out. 

1. Frank, M. G. & Benington, J. H. The role of sleep in memory consolidation and brain plasticity: dream or reality? Neuroscientist 12, 477–488 (2006).

2. Walker, M. P., Liston, C., Hobson, J. A. & Stickgold, R. Cognitive flexibility across the sleep–wake cycle: REM-sleep enhancement of anagram problem solving. Cognitive Brain Research 14, 317–324 (2002).

3. Newbury, C. R., Crowley, R., Rastle, K. & Tamminen, J. Sleep deprivation and memory: Meta-analytic reviews of studies on sleep deprivation before and after learning. Psychol. Bull. 147, 1215–1240 (2021).

4. Xie, L. et al. Sleep drives metabolite clearance from the adult brain. Science 342, 373–377 (2013).

5. Plog, B. A. & Nedergaard, M. The Glymphatic System in Central Nervous System Health and Disease: Past, Present, and Future. Annu. Rev. Pathol. 13, 379–394 (2018).

6. Aalling, N. N., Nedergaard, M. & DiNuzzo, M. Cerebral Metabolic Changes During Sleep. Curr. Neurol. Neurosci. Rep. 18, 57 (2018).

Cover photo by Isabella Fischer on Unsplash

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