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I've seem several articles claiming that sleep is akin to the usual proccess of hard drive defragmentation, however for your brain. This process allegedly prunes synapses/memories and reconsolidate memories.

You can find several versions of this claim:

(..) So the brain must periodically clear out the synaptic underbrush—analogous to “running a repair-and-cleaning program on your computer to defrag the hard drive,” says psychologist William Killgore of Harvard Medical School. http://time.com/4737596/sleep-brain-creativity/

(...)A defrag consolidates the same data into a more logical order. Defragmentation is a taxing chore for the computer, so many people schedule it to happen overnight. In the same way, sleep may serve to reorganize and reconsolidate memories. http://blogs.discovermagazine.com/neuroskeptic/2011/08/21/is-sleep-brain-defragmentation/#.XIg-xbjQ-Uk

(...) During sleep, your brain goes into a cleansing and restorative mode. It washes away all of the unimportant and unnecessary thoughts and memories to maintain space for all the memories you need to function, and reorganizes the free space for new memories you’ll make tomorrow. In a nutshell, your brain gets refreshed when you sleep. And this process is so important because it aids in the healing of dementia. http://handsoffmybrain.com/2018/09/defrag-your-brain-with-sleep-but-what-do-you-do-if-you-cant-sleep

Does the brain do something similar to defragmentation while we sleep, by cleaning synapses and "washing away" unimportant thoughts?

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    What sort of evidence would it take to convince you it was or wasn't a "valid analogy"? I could argue that human memory isn't like a disk drive, in that it doesn't assign specific neurons to specific memories, so defrag analogy is invalid, or I could argue that there is a "consolidation" during the REM cycle, so the defrag analogy is helpful. Is this not just opinion-based?
    – Oddthinking ♦
    Commented Mar 12, 2019 at 23:55
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    @Oddthinking I'll edit that out, if it is causing problems. I don't see it as opinion based - there are several places saying "X", and I'm asking if "X" is true.
    – T. Sar
    Commented Mar 13, 2019 at 0:29
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    'there are several places saying "X"' - I don't think any are saying "A is equal to B". They are saying "A is analogous to B." An analogy isn't true or false - it is helpful to understanding or not helpful to understanding.
    – Oddthinking ♦
    Commented Mar 13, 2019 at 1:14
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    Seems like a great analogy to me as it explains a function of sleep in familiar terms. But it is an abstraction, which you are treating as concrete in your question. Steven Novella has written some articles on the subject. I like this one, which shows how sleep deprivation can have an adverse effect on memories: theness.com/neurologicablog/index.php/sleep-and-false-memory
    – Jerome Viveiros
    Commented Mar 13, 2019 at 8:20
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    I think the criticism is that you are taking the analogy too literally. It's a good analogy in many ways, but not meant to be taken literally. A hard drive never forgets in the first place, unless defective or ordered to delete data. A defrag on a hard drive is solely for faster access. The brain operates fundamentally differently, but it also has this function that works much better during a time of low activity, and which also is "reorganizing" in a way. It's a good analogy, IMHO, but your question "is it true?" is from a false premise. No-one actually made the claim you're skeptical of.
    – DevSolar
    Commented Mar 13, 2019 at 13:11

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There is a plethora of scientific work that all points towards sleep being very important for the kind of cleanup and reorganisation you are referring to.

Information like this is readily available e.g. at WP:Sleep#Functions. I've enriched this with a couple things I found in the German Wikipedia, not linked from the above "Sleep" page:

1) The brain / glymphatic system is much more efficient at chemical "clean up" during sleep.[1]

A publication by L. Xie and colleagues in 2013 explored the efficiency of the glymphatic system during slow wave sleep and provided the first direct evidence that the clearance of interstitial waste products increases during the resting state. [...] Xia and Nedergaard demonstrated that the changes in efficiency of CSF–ISF exchange between the awake and sleeping brain were caused by expansion and contraction of the extracellular space, which increased by ~60% in the sleeping brain to promote clearance of interstitial wastes such as amyloid beta.

2) Sleep promotes branch-specific formation of dendritic spines after learning.

These findings indicate that sleep has a key role in promoting learning-dependent synapse formation and maintenance on selected dendritic branches, which contribute to memory storage.

3) Sleep inspires insight.

Insight denotes a mental restructuring that leads to a sudden gain of explicit knowledge allowing qualitatively changed behaviour. [...] Sleep consolidates recent memories and, concomitantly, could allow insight by changing their representational structure. Here we show a facilitating role of sleep in a process of insight.

At which point I stopped collecting more references, simply because there are so many and the claim is thoroughly confirmed.

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  • Thank you for your answer. I find the first article (the one about chemical cleanup) specially interesting. This is exactly what I was looking for.
    – T. Sar
    Commented Mar 13, 2019 at 14:24
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    @T.Sar The chemical cleanup hypothesis of sleep is possibly quite different from the process you described in your OP as "prunes synapses/memories and reconsolidate memories" - I would say in the field right now they are essentially opposite hypotheses of the purpose of sleep. That said, I don't think they are actually mutually exclusive but it would be a mistake to consider them synonymous.
    – Bryan Krause
    Commented Mar 13, 2019 at 16:00

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