This article is an excerpt from the Shortform summary of "Why We Sleep" by Matthew Walker. Shortform has the world's best summaries of books you should be reading.
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How are sleep and memory related? Does lack of sleep hurt your memory, and can more sleep improve it?
Sleep and memory are closely linked. When you miss out on sleep, your memory function can be impaired. Sleep and memory are so closely connected that you may even experience chronic memory loss as a result of a lack of sleep.
How Sleep and Memory Work
What do sleep and memory have to do with each other? Do the effects of sleep on memory have a lot to do with your daily life?
Science says yes. Getting good sleep improves your brain in these ways:
1) Sleep improves long-term factual recall
Your brain stories different memories in different places. The hippocampus stores short-term memory with a limited capacity; the cortex stores long-term memory in a large storage bank.
The slow-wave, pulsating NREM sleep moves facts from the hippocampus to the cortex. This has two positive effects: 1) it secure memories for the long term, and 2) it clears out short-term memory to make room for new information and improves future learning.
Have you ever woken up recalling facts that you couldn’t have recalled before sleeping? Sleep may make corrupted memories accessible again.
While good sleep improves memory, sleep deprivation can prevent new memories from being formed. This might be partially because the hippocampus becomes less functional with less sleep, partially because lack of NREM sleep prevents solidifying of new memories.
Unfortunately, making up a sleep deficit later doesn’t help recover a previous days’ memory – if you lost it, you’ve lost it. That’s why sleep and memory are so important.
2) Sleep prunes memories worth forgetting
Sleep doesn’t preserve all memories equally strongly – somehow, the brain knows which memories are useful and worth preserving, and which ones are useless and OK discarding.
Experimentally, this has been shown in experiments where subjects are given a list of words and instructed which words to remember and which to forget. Students who get to take a nap show stronger memories for the appropriate words, compared to students who don’t nap.
3) Sleep increases “muscle memory” or motor task proficiency
You might struggle with a motor task (like playing a tough sequence on piano), but after sleeping, be able to play it flawlessly. Sleep seems to transfer motor memories to subconscious habits.
Sleep deprivation also worsens general athletic performance: getting less sleep decreases your aerobic capacity, time to exhaustion, and recovery; and it increases risk of injury and lactic acid generation.
The above benefits generally occur in NREM sleep, which is concentrated in the beginning of sleep. In experiments, participants who have NREM sleep disrupted perform worse than those who have REM sleep disrupted. Consider this as one of the effects of sleep on memory.
A few last scientific details:
- Motor memory is associated with stage 2 NREM, which is concentrated in the last cycle of sleep.
- Sleep spindles are associated with better memory effects.
Now that we understand the impact of sleep on the brain, imagine how we can apply this knowledge into useful therapies:
- Imagine modifying sleep to selectively control what to remember from the day – like remembering content for an upcoming test and a happy moment, while decreasing traumatic moments.
- (This might lead to the side effect of having a have distorted memory of life).
- Use sleep to delete traumatic memories (for PTSD) or train away bad motor habits (like substance abuse).
- Find ways to augment the natural abilities of sleep, like with electrode stimulation of brain, pulsing sound in sync with brain waves, or rocking the bed rhythmically.
Does It Lead to Alzheimer’s?
While no definitive causal link has been shown yet, some theorize that the effects of sleep on memory may have something to do with Alzheimer’s. Sleep losses may contribute to Alzheimer’s through a few mechanisms:
- Frontal lobe degeneration (especially through Alzheimer’s characteristic amyloid plaques) disrupts NREM sleep.
- Lack of NREM sleep disrupts memory formation, a key symptom of Alzheimer’s.
- (Notably, the hippocampus is not affected by amyloid plaques, presenting a conundrum to scientists on why memory is disrupted in Alzheimer’s.)
- Lack of NREM sleep disrupts the lymphatic cleanup system, during which glia shrink to less than half their normal size and amyloid plaques are cleared out more readily.
It’s easy to see how a vicious cycle can occur – frontal lobe degeneration disrupts NREM sleep, which causes further frontal lobe degeneration.
Sleep loss precedes Alzheimer’s by several years, suggesting this could be an early diagnostic. Sleep and memory could be contributing factors.
Encouraging NREM sleep, through artificial brain stimulation if needed, might be therapeutic for Alzheimer’s. It could also be prophylactic, the same way statins protect against heart disease.
Sleep and memory certainly need more study, but they’re obviously linked. When you are working on improving your sleep, remember that sleep and memory are connected, and improving one improves the other.
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Here's what you'll find in our full Why We Sleep summary :
- Why you need way more sleep than you're currently getting
- How your brain rejuvenates itself during sleep, and why nothing can substitute for sleep
- The 11-item checklist to get more restful sleep today