The brain is one of the vital organs of the body. Human brain consumes about a fifth of total energy, although not large in size. Brain activity and energy expenditure fluctuate throughout wakefulness and sleep. The brain’s energy consumption during sleep is smaller, although not equal through sleep stages – the energy use is the highest in the REM stage of sleep.
Recently it was discovered that energy levels spike while brain activity hits a low soon after falling asleep. The increase in energy levels was observed in those brain areas which are the most active in wakefulness. This could be linked with cell restoration.
Where does the brain’s energy come from? About Glucose and ATP
Just like all cells in our body, the brain gets its energy from food. Glucose is a type of sugar which we get by breaking down the food we eat. As cell metabolism continues to break it down, glucose gives us heat energy and ATP.
Adenosine triphosphate (ATP) is a molecule which also contains energy, although about 90 times less than glucose. The energy in ATP is either stored or transferred – and in ATP, it’s a lot quicker for energy to be passed from one cell to another than glucose. This is why scientists call it the “energy currency” of cells. The energy contained in ATP is the only form of energy our body is able to use.
Energy use in sleep
When we sleep, our body consumes about 5-10% less energy as metabolism, heartbeat, and breathing slow down and body temperature decreases. The overall energy decrease gave rise to the hypothesis that we sleep in order to conserve energy.
Although we burn calories while sleeping, it is not easy to determine how much, as it depends on room temperature, body mass, fitness, age, sleep quality and more factors.
Energy use through sleep stages
It is known that there are four stages of sleep and that they are characterized by different types of brain activity. A study has shown that sleep stages don’t influence energy expenditure throughout the body, although others claim that more energy is consumed during REM sleep, compared to other sleep stages.
But what about the brain? It seems that the brain during sleep uses less and less energy as it goes into deeper stages. In NREM stages, light sleep requires more glucose consumption than deep sleep (slow-wave sleep or SWS). However, as REM sleep is characterized by more activity, it requires more energy.
A study aimed to find how much non-REM sleep energy consumption differed in depressed individuals compared to those of good mental health. The results showed that depressed brains are up to 44% more metabolically active in light sleep and SWS than non-depressed ones. Amygdala, the fear and emotion center of the brain was among the most active areas.
The brain’s energy expenditure at rest
Even at rest, the brain uses a lot of energy. If given a task, energy consumption only slightly increases. Scientists have long thought of this constant energy consumption as “noise” with no particular use. A study from 2015 has shown why that “noise” exists.
Certain brain areas, distant physically, but together involved in certain tasks, have been found to preserve their shared activity even when a person is resting or sleeping. These brain areas are called the default mode network (DMN) because they are active when we are not doing anything. When some activities are performed, DMN shuts down.
The scientists implanted electrodes in human brains for medical purposes. Although the main reason for this was to pinpoint the place from which epileptic seizures in the patients began, the scientists were also able to look at DMN and compare its activity in task-performing, resting and sleep. They found that a similar level of activity persists in all the states. As an explanation to why the brain needs so much energy for something it doesn’t use, study authors say that the brain might be maintaining relationships and preparing itself for the future when the action is needed.
Restoration of the brain’s energy
A study from 2010 a lot of raised interest when it was published. For the first time, scientists discovered a “surge” in ATP levels in rat brains in the first part of night’s sleep. They link this surge with cell restoration because the ATP-level-high brain areas were those that are active in wakefulness. They are frontal cortex, basal forebrain, cingulate cortex, and hippocampus.
The surge only occurred in NREM sleep (non-rapid eye movement), at the same time as the cell activity in the mentioned areas decreased. To test whether ATP surge depends on sleep itself or solely the time (part of the day), the rats were kept awake by gentle handling. When sleep deprived, they had no ATP surge.
As brain cells consume a lot of energy in wakefulness, they need rest and restoration, so they lower the activity, and fill up with ATP which probably powers the restorative processes necessary for healthy functioning of the brain.
The ATP surge was seen to occur simultaneously as the slow-wave sleep. The findings are very interesting and show how and when the energy is distributed in order to repair and restore nerve cells.
Sleep quality, sleep deprivation, and energy
When we are awake, our brain cells work, metabolize, send out signals, and create new synapses. The more we are awake, the more sensitive our brain gets. The neurons (nerve cells) need to take a break, clear our from adenosine (the by-product of neurons), and get more energy.
This can only be done during sleep because even when we sit still, our brains consume a lot of energy and, as described above, our nerve cells keep working. Many people tend to increase their calorie consumption when they are tired. Although we do turn food into energy, eating cannot give us the restorative energy – only sleep can do this.
Gathering all data presented in this article, we would get the following – deep sleep is sensitive to stress levels. If we are depressed, we are likely to get non-restorative sleep, because our increased brain activity and burden our brain endures will continue throughout the NREM sleep.
Similar goes for sleep-deprived individuals – they are not able to fall asleep quickly and have the deep, quality rest needed for the energy spike and restoration.
Our brain cells need some quiet time during deep sleep in order to be fresh and ready for new challenges the following day.
- Foster B. L, RangarajanIntrinsic V, et al. Task-Dependent Coupling of Neuronal Population Activity in Human Parietal Cortex. Neuron. April 8, 2015. https://www.cell.com/neuron/fulltext/S0896-6273(15)00214-7 Accessed February 26, 2019.
- Ho A. P, Gillin J. C, et al. Brain glucose metabolism during non-rapid eye movement sleep in major depression. A positron emission tomography study. Archives of general psychiatry. July 1997. https://www.ncbi.nlm.nih.gov/pubmed/8660131 Accessed February 26, 2019.
- Sleep and Brain Energy Levels: ATP changes during sleep. Journal of Neuroscience. December 1, 2010. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2917728/ Accessed February 26, 2019.
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