Study Says The Purpose Of Sleep Is To Forget

by Aby Nicole League

Sleep is a biological need. Harvard Medical School regards it as one of the pillars of health together with nutrition and exercise. The benefits of sleep cannot be understated. A long line of studies show that our nightly zzz’s are essential in tissue repair, muscle regeneration, and mental health.

Harvard scientists compare the function of sleep to eating: “Just as eating relieves hunger and ensures that we obtain the nutrients we need, sleeping relieves sleepiness and ensures that we obtain the sleep we need.”

But what is the real purpose of sleep? Some experts say that it clears the brain of cellular wastes, others claim that it recharges the body after a long day. Two papers, released earlier this month, offer new explanation on why we doze off.

Past Theories on the Purpose of Sleep


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The adaptive or evolutionary theory suggests that organisms sleep as survival mechanism against external harm. Animals, specifically preys, are able to protect themselves from being killed by predators by staying still throughout the night. The behavioral strategy is believed to have evolved to what we now know as sleep.

According to the energy conservation theory, people sleep to reduce energy demand and use especially during periods least efficient to search for food. Early findings show that energy metabolism is reduced by as much as 10 percent in humans during sleep. Energy conservation is greater among other species. The energy conservation theory is partly related to the adaptive theory.

The restorative theories claim that sleeping for hours help the body repair and rejuvenate itself. The body’s restorative functions such as muscle growth, tissue repair, and growth hormone release mostly occur when we are asleep. Another aspect of this theory involve our cognitive function. The brain produces adenosine when we’re awake. The build up of this nucleoside, a by-product of brain cells’ activities, is believed to promote our perception of being tired. “During sleep, the body as a chance to clear adenosine from the system, and, as a result, we feel more alert when we wake,” explains Harvard sleep experts.

The Brain Plasticity Theory

The brain undergoes changes in its structure and organization during sleep, according to the brain plasticity theory. This theory is among the most recent and compelling explanations on the purpose of sleep.

The neural connections that form our memories are strengthened, boosting memory consolidation. Thus, people unable to meet the suggested seven to nine hours of sleep each night are likely to suffer from memory loss and cognitive impairment.

Sleeping to Forget

father sleeping with infant

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Two research papers published in the journal Science suggests that people sleep to forget. The first paper by University of Wisconsin-Madison biologists Giulio Tononi and Chiara Cirelli claims that sleep helps in rebalancing brain synapses that get “noisy” in the morning. “Sleep is the perfect time to allow the synaptic renormalization to occur because when we are awake, we are ‘slaves’ of the here and now, always attending some stimuli and learning something,” said Cirelli.

For the study, the scientists analyzed the brains of mice: four that had just slept, four that had been kept awake to play with toys, and four that stayed awake on their own. High-resolution three-dimensional images of thousands of brain synapses were examined to calculate the contact between nerve terminals and dendritic spines, or the receiving ends of brain synapses.

The findings show an 18% overall decrease in the number of synaptic connections in the mice that had slept. Interestingly, the downscaling spared the larger dendritic spine involved in learning. “That there’s such a big change overall is surprising,” said Tononi.

Brain Synapses Shrink During Sleep

brain synapses

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The second study looked deeper into the downscaling of synapses during sleep. Graham Diering and his colleagues at Johns Hopkins University assessed two-photon images of synaptic proteins in the brains and dendritic spines of live mice. The scientists found that the number of surface proteins in mouse brains dropped during sleep, suggesting that their brain synapses were shrinking.

The results also showed that synaptic scaffold protein called Homer1a enters dendritic spines to disassemble receptors and their molecular partners. The role of Homer1a protein in triggering synaptic downscaling was further confirmed in genetically-engineered mice that lacked the gene. The scientists found that the protein composition of the dendritic spines of these mice did not change when they slept.

Homer1a: Triggers the Brain’s Pruning Machine

homer1a gene

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How is the Homer1a gene switch on the pruning machinery crucial in learning? Diering and his colleagues gave regular mice a memory test by putting them in a room where they were given a mild electric shock is they cross one section.

In the evening, the scientists injected a chemical into the brain of some of the animals that blocked neurons from pruning their synapses. In the following morning, the mice with injected chemicals froze fearfully recalling the shock even when they were placed in a different room.

The scientists theorize that without the pruning during sleep, memories end up fuzzy in the morning.

The Brain Knows What to Forget

Tononi’s team said that a fifth of brain synapses were unchanged during nighttime pruning, suggesting that some synapses have well-established memories that cannot be altered. “You can forget in a smart way,” Tononi remarked.

Getting the Sleep You Need

healthy food for sleep

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There will be more studies exploring sleep, memory, and their relations with each other. Nevertheless, the well-established benefits of quality zzz’s should be enough to encourage you to make lifestyle changes. How can you acquire better sleeping habits? Check your daily activities and sleep routines.

Eating huge portions of unhealthy food such as fast food meals can make falling asleep harder because of stomach discomfort. Doing strenuous exercise in the late evening or right before bedtime can also cause sleep problems. Moreover, studies show that anxiety results to sleeping problems and insufficient sleep conversely leads to an anxiety disorder. It’s important to keep your stress levels in check for your sleep health.

We have been investigating the purpose of sleep since time immemorial. Early theories suggest that sleep evolved from the behavioral strategy of preys keeping still throughout the night to avoid predators. This is supported by findings that sleep is a way of conserving and restoring energy. The advent of technology has enabled us to investigate further by studying the brain. Today, scientists have presented another explanation on why we sleep: to forget certain things in our waking life.

(Aby is a qualitative researcher and a passionate writer.  She writes mostly about health, psychology, technology, and marketing. You can follow her at @abyleague.)

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