In this article, we will cover the energy consumption of the human brain during sleep hours, as well as where that energy comes from and related information. Along the way, we may mention several experiments on non-human species that led us to draw solid conclusions that apply to humans.
Despite all of humanity’s research efforts, we still don’t know everything there is to know about the human brain. So many mental illnesses (and sleeping disorders, for that matter) have unclear causes and lack a reliable means of therapy. However, the question of its energy consumption is one we have at least partial or vague answers to. For an average adult in a resting position, the brain takes up around 20% of total energy. It may sound absurd given its small size compared to the rest of the body, but the primary purpose of the brain (processing and transmitting information through the use of electrical signals) is quite taxing on our energy reserves. As much as 75 percent of the brain’s energy expenditure goes towards synapses (the “communication pathways” between neurons and either other neurons or cells).
In this article, we will cover the energy consumption of the human brain during sleep hours, as well as where that energy comes from and related information. Along the way, we may mention several experiments on non-human species that led us to draw solid conclusions that apply to humans. The more you know about your sleeping energy consumption, the better you can plan your diet and overall lifestyle to stay healthy and improve your sleep quality.
An awake adult uses about 20% of their total energy on brain function, when all the values are averaged out, of course. A more detailed look reveals that 15 percent of our cardiac output (the amount of blood your heart pumps out per minute – for people weighing in at 70kg, this is roughly five liters per minute during resting) goes towards brain activity. At least 20 percent of our total oxygen supply fuels brain activity as well.
Additionally, 25% of all glucose utilization goes towards powering the brain. Glucose is possibly the most crucial energy source in our body, and it is stored primarily in our liver and skeletal muscles as glycogen, its polysaccharide form. Low blood sugar or low water levels are frequently treated with dextrose solution, which is a combination of glucose and water. This solution is on the World Health Organization’s Essential Medicines List (EML), a resource on the most practically useful medicines for a basic health system.
Depending on which sleep stage we’re in, our body distributes energy in different ways. During stage 2 and 3, meaning light and deep sleep respectively, our brain is far less active than while we’re awake. Meanwhile, the REM stage is characterized by brain activity with a close resemblance to when we’re awake.
There’s a common misconception floating around in layman conversations that our brain uses up more energy when we’re solving a particularly difficult task, or “thinking hard.” The truth is, the intensity of the task doesn’t affect the amount of energy the brain requires but depending on what you’re doing, different parts of your brain demand a higher percentage of the overall energy “bandwidth.” For example, if you’re trying to hold a conversation, then the area of the brain responsible for speech and forming sentences will spark into action. This localized energy increase isn’t massive – clocking in at around 8% extra energy at most. The more stimulation you send towards a specific area of your brain, the EEG delta power is higher in that area once the person enters NREM sleep, specifically stage 3 or as it’s normally called – deep sleep.
Many theories are circulating in the scientific world about why and how sleep as a process evolved in animals, including humans. One largely discarded theory (called the Inactivity Theory) explains how sleep was originally meant to put us in a sort of “stasis” during the time of day where roaming around outside would be risky due to the increased threat of dangerous predators. This self-preservation mechanism would force us to stay put in whatever hideout we found, even against our judgment. While this theory was rejected due to various gaps in its logic (such as “Why would we want to be unresponsive to the environment and immobile if we’re in danger from predators?), a similar idea has sprung up, called the Energy Conservation Theory.
This theory suggests that because humans of old couldn’t compete with the apex predators of their environment at night, they would prefer to search for sustenance during other parts of the day. The competition for energy sources (primarily food) would force them to rest and save their energy until they can resume their hunting and forage more safely when the most threatening carnivores are asleep or otherwise inactive. The evidence for this idea lies in how our metabolism changes during the night. While we’re asleep, body temperature is considerably lower, heart rate and breathing slow down. It is estimated that our metabolism as a whole is around 10% slower than while we’re awake. Therefore, it is easy to conclude that sleep evolved in humans (and other animals) at least partly as a way to preserve energy during a time of day when it is difficult or too risky to find sustenance. This theory does not discredit all the other research pointing out the physically and mentally restorative properties of sleep.
Overall, studies show that the total energy expenditure (or EE for short) of our body doesn’t drastically change between sleep stages. Here’s an example that can help prove that – while our muscles are not active at all during the REM stage (as they’re essentially paralyzed), the brain’s increased activity makes up for that, evening out the energy usage. Conversely, the NREM stage boasts higher energy expenditure on our muscles while we switch positions (especially if the person suffers from periodic limb movement disorder or a similar condition), but the brain is less active overall.
On a similar note, scientists have discovered that sleep deprivation increases energy expenditure while we’re asleep. People dealing with sleep deprivation often file subjective reports about feeling cold. However, it’s hard to confirm whether this is simply because fragmented sleep leaves us in a waking state more often (as we expend considerably more energy while we’re awake) when we’re dealing with a noisy environment or one that is unsuitable in some other way. This study serves as further supporting evidence for the idea that one of the reasons sleep appeared in our evolutionary path is to conserve energy until we can find more food at a safer time of day.
One of the most revolutionary diagnostic techniques we can rely on during sleep research is positron emission tomography (or PET for short). It is deep in nuclear medicine territory, as it uses a radionuclide (a purposefully-created unstable atom that indirectly creates gamma rays) that is introduced into the body through the use of a radioactive tracer (a chemical compound where an atom has been replaced by a radionuclide for use in research and diagnosis). This method is used to monitor and track various metabolic processes by creating a detailed 3d model of the entire body for scientists to examine.
Let’s put it in layman’s terms: positron emission tomography is used for sleep research to discover which parts of the brain consume the most glucose during a given period while we’re resting. It is presumed that these parts of the brain are the most active, as they draw on our energy reserves the most. During REM sleep, for example, the most active and energy-hungry parts of our brain include the pontine tegmentum, the back portion of the cortex and the thalamic limbus. On the other hand, the prefrontal cortex and parietal lobe are dormant and consume way less energy.
New ideas and theories are proposed quite often in the world of sleep research. One of the most groundbreaking discoveries happened when scientists realized through experimentation that not every part of the brain rests at the same time and that different parts of the brain experience different sleep intensities. It isn’t a trait that’s specific to us as humans, mind you – other species have shown similar behavior during experiments. If you hook up a dolphin to an EEG, the readings will show that their cerebral hemispheres don’t exhibit high-amplitude delta waves at the same time during NREM sleep. The blood distribution to our brain is different between REM and NREM stages, solidifying the idea that different parts of the brain rest at a different time and intensity.
An example that’s been talked about is that of sleepwalkers. Sleepwalkers are often considered to be both asleep and awake at the same time. They retain the ability to move around objects and navigate their immediate surroundings similar (although not as effective) to a waking person, but they’re as unresponsive to various external stimuli as a fully asleep person. If every part of the brain rested and was inactive at the same time, this would not be possible.
By looking into the sleeping and awake state of individual cortical columns (which are thought to be the brain’s basic processing unit) in rats, it was discovered that each individual column switches from an awake to a sleeping state individually (you can visualize it as an “on-off switch”). If the rat as a whole is sleeping, a vast majority of these cortical columns were in their own sleeping state, but not all of them. The same applies when the rat is awake. While most of the columns show signs of being in the waking position, columns in a sleeping state can still be found and measured.
The behavior of these cortical columns is connected to the homeostatic process. As a rule, the longer an individual cortical column has spent in the waking state, the more likely it is to switch to a sleeping one. Following the rat example, if you make a test where the rat is trained to lick as a response to the stimulation of one of its whiskers, this response will be unreliable if the cortical column that is meant to process that stimulation is asleep. As a result, it is believed that a cortical column is the smallest brain unit that can exhibit sleep-like behavior. While these experiments were primarily done on rats, similar findings were discovered with humans involved, suggesting that this is at least a trait consistent in mammals, if not most animals in general.
This study process also contributes to the idea that NREM and REM sleep developed together as a way of helping the brain recover by allowing different parts of the brain to rest at different times. Given how the primary roles of these two stages differ in the sense of bodily and mental repairs, it’s not hard to see how this theory holds water.
There are no reported cases of complete insomnia after the patient experiences a stroke or other kind of brain lesion (and over a million brain lesion victims were inspected, so the numbers can’t lie at this point). If they survive as a whole, a sleeping rhythm is re-established among the surviving groups of neurons despite the overall damage caused to the brain. It leads scientists and sleep researchers to believe that sleep is a property of individual neuron groups, not necessarily of the brain as a whole. Additionally, if the blood supply is limited to any part of the brain, neurons will immediately shut off to maintain a base level of brain operation.