How Sleep Helps Your Memory

One of the main theories about how memory works in the brain has a new bit of proof. The idea is that the brain's synapses somehow consolidate all the new information they've learned during the day while we slumber, setting some of it aside in permanent memory. The problem scientists have encountered with this theory is pretty simple: how the hell does the process of memory consolidation work, and why is sleep the right time to do it?

A new study just released in Science by researchers at Wisconsin-Madison has a good explanation for this. According to a four-year study of the synapses of mice, it turns out that the brain actually prunes itself during sleep. Our synapses grow intensely during the day as they're stimulated with lots of new information and potential memories, but over the course of a good night's sleep, they shrink by up to 20 percent. The shrinking wasn't random, though: the most "stable" synapses, making up about a fifth of the potential bits to be pruned, were left alone. The scientists think the process is how the brain consolidates certain memories while forgetting other, useless information to prevent total overload and enable the brain to receive more stimuli the next day. Our brains, it turns out, aren't an infinite resource; they're constantly renewing themselves to try and calibrate the stuff we've learned during the day, and choosing which bits to eliminate and which to "save" as memories.

Another bit of new science also lays out precisely what happens when we don't get enough sleep, and why that makes our memories go fuzzy. Researchers at Johns Hopkins have just released a cool bit of research into the brain's synapses. They found that that specific proteins are responsible for the "signalling" around memory and sleep, and that interfering with them is what causes our memory difficulties after pulling an all-nighter.

Somewhat awkwardly, the Johns Hopkins study made a lot of the same discoveries as the Wisconsin-Madison research — but it takes a different angle. They came to the same conclusions about the brain pruning itself overnight and why that's essential for memory. But they also found that the pruning process is controlled by a protein called Homer1a (yes, scientists read the classics too). It builds up in the synapses of sleeping mice and appears to have a big role in "scaling down" the brain every night.

Homer1a isn't just an interesting factoid, though. The researchers also found that caffeine and other stimulants interfered with the brain's production of Homer1a during sleep; and they also suggested that sedatives of the benzodiazepine family get in the way of Homer1a production too. Either option is bad for the brain's pruning work overnight, and may make people wake up confused.

Not all sleep aids might be bad for memory, though, and the reason for that, according to research from Californian scientists in 2013, is "sleep spindles:" spikes of brain activity that happen in the hippocampus as we sleep. (Spindles are the pointy bits on spinning-wheels, like the one with which Sleeping Beauty pricked herself. The activity spikes look like spindles on brain-activity monitors, hence the name.) It seems that as we snooze, the hippocampus's processing and consolidating of memory causes it to have sudden surges of activity. Sleep spindles only really happen in stage 2 sleep, though we can reach that stage in daily naps (which is why they're likely extremely good for your memory).

The interesting thing about sleep spindles, though, is that certain sleep aids seem to enhance them. The class that seems to be promising is nonbenzodiazepines. The Californian scientists gave 49 different people either Ambien, Xyrem, or a placebo, then tested their sleep quality and did some memory tests once they woke up. The results were intriguing: Ambien makes sleep spindles denser, and also improved peoples' verbal memories when they woke up. They're still not entirely sure why Ambien had this particular affect, but it's an interesting piece of information to have if you're selecting sleep aids.

Here's something distinctly odd to know about the whole memory-sleep relationship: it's actually a two-way street. We mostly conceive of the brain as consolidating memories as we slumber, as a kind of side effect, but it turns out that the process is so important to the body that memory processing might actually send us to sleep. It's a strange phenomenon, so let's get into it in a bit more detail.

Back in 2015, scientists at Brandeis University discovered that fruit fly brains have an interesting reaction to memory. They monitored a bit of their brain similar to the human brain's hippocampus, where a lot of the synapses related to memory are located. It turns out that when the fruit fly is consolidating its memories, it actively becomes sleepier. As it starts converting information into long-term memory, it encourages the fruit fly to start snoring to make the process easier. If this also applies in humans, it means that part of our desire to nod off might come from our brains pressing the shut-down button to deal with memories properly. Which is as good a reason as any for feeling sleepy in a particularly intense history class.

So there you have it — nap away. It's good for your grades.