Z: The most important letter in the alphabet

by Lindsay Miller on September 28, 2012

Okay, so it’s not the most important letter of the alphabet, but one thing is certain: most of us could probably use more “Z’s” in our life. That’s right—sleep! Although it varies by person, the recommended amount of sleep for adults, according to the National Sleep Foundation, is 7-9 hours. Some (myself include) may chuckle at this recommendation, as a 9-hour-snooze is literally something we can only dream of.

Source: FreeDigitalPhotos.net

But why do we need sleep? I had a friend ask me this question recently. Like a battery, I said, we need to recharge so that we have energy for the next day. You know, we have to re-wire. I stopped myself. Frustrated by my inability to explain sleep without analogizing our nervous system to familiar technology, I set out to find an answer: what is really going on when we sleep?

We know all too well the consequences of what happens when we don’t sleep—delirium, irritability, excessive yawning, multiple trips to Starbucks—but what happens when we do sleep is a question that scientists are still grappling with. Some of the more popular theories about the function of sleep include energy conservation and bodily restoration, but one of the more prominent areas of research today has to do with the role of sleep in memory consolidation.

Memory consolidation can be thought of as the reactivation and reorganization of neural pathways created by events and experiences (short-term memories) into more “retraceable” or “memorable” pathways (long-term memories).

A 2011 Psychological Research study found that while we are enjoying our alleged “rest,” our brain is actually hard-at-work sifting out unimportant memories and reinforcing more important ones. Yup, you read that right: our brain shows preference for memories that are somehow relevant for our future expectations and plans. When subjects of the study were given information and told they would have to retrieve that information later, they were more likely to remember it after a period of sleep than were subjects who were given the same information but not expected to retrieve it. Of significance is that these groups were equally able to retrieve the information during wake intervals–the differences in memory retention emerged only after the sleep period. Strengthening the connection between slow-wave sleep, specifically, and memory consolidation was the finding that those who were expected to retrieve the information had noticeably increased slow-wave brain activity during their post-learning snooze!

So sleep helps us to remember (and forget)…but that’s not all, folks.

An even more recent study done at Northwestern University revealed that previously learned skills and information can actually be reinforced or “practiced” during slow-wave sleep. Prior to a nap, participants performed equally well on two previously-learned musical sequences. After their nap, study participants performed with greater accuracy on the sequence that researchers had played for them quietly during their slumber. Though this study only looked at musical sequences, researchers are hopeful that this effect might hold for the reinforcement of other skills and processes, such as language acquisition.

Somehow I don’t think that falling asleep on top of my organic chemistry book all those times in college had the same effect…but the message still rings clear. Sleep is of primary importance in healthy mental functioning, and committing to getting those extra few hours could make a big difference!

Having trouble falling asleep? Sick of counting sheep? Try reading this blog 5 times, and that should do the trick!




Born, J., & Wilhelm, I. (2011). System consolidation of memory during sleep. Psychological Research, 76(2), 192-203.

Antony, J. W., Gobel, E. W., O’Hare, J. K., Reber, P. J., & Paller, K. A. (2012). Cued memory reactivation during sleep influences skill learning. Nature Neuroscience, 15, 1114-1116.


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Margaet September 28, 2012 at 10:47 am

Great post! I always tell my students that the best thing they can do the night before a test is sleep. I don’t know if they believe me but now I have more sources to back me up. Great Topic! Fairly easy reading. What exactly is slow wave sleep? Again important topic to all of us who probably don’t get enough sleep.


Lindsay Miller September 28, 2012 at 2:42 pm

Thank you for your feedback, I am glad you enjoyed the post! Slow-wave sleep is essentially deep, dreamless sleep. It is characterized by slower “delta” brain waves and no rapid eye movements. It is very difficult to wake someone from their slow-wave slumber!

As Ginny pointed out below, providing a definition in the post would have been helpful. Something I’ll keep in mind next time!

Margaret October 2, 2012 at 3:57 pm

Lindsey, Thanks for clearing that up.

Quintus September 28, 2012 at 10:48 am

Interesting post. Some research also shows that a nap at lunchtime does the body and brain good for the afternoon session. I knew someone at work who had a very old office in an old building. There were huge cupboards horizontally on top of bookshelves etc. He turned then into a nest and slept there every lunchtime and this for years.
No wonder I could never find him.

Ginny Kendall September 28, 2012 at 12:14 pm

I loved your post! Judging from the number of “sleep aids” advertised on TV, it is a problem for lots of folks. (me, included). Our society is so “always on” it’s hard to wind down. You provided a link to the definition of slow wave sleep, which I read, but perhaps you could have just defined it in your article by saying it is a period of dreamless sleep.
You included just enough humor to make it fun to read! I look forward to your next post.

Lindsay Miller September 28, 2012 at 2:46 pm

Definitely good advice with regards to supplying a definition–I will keep in this in mind in the future. It’s always a challenge walking the line between being concise and being thorough!

Rhonda September 28, 2012 at 4:37 pm

Great topic, very relevant. I’m wondering if there are any recommendations based on the studies about people focusing on high-value information right before they go to sleep? Are they more likely to experience effective memory consolidation if the sleep interval is in close proximity to creating the relevant memories?

Also, do you have a sense of whether the length of the sleep interval is important? That is, does it matter whether it’s a nap or a full night’s sleep? I recently read that studies have shown the 8-hour sleep period is not necessarily best in a single chunk.

Lindsay Miller September 30, 2012 at 11:45 pm


These are some great questions! I wish I could have explored these more in my post.

I will preface with this: so much of what scientists know about memory and sleep is still unknown territory. However, there have been a few studies looking at the relationship between timing of learning, sleep, and memory retention. One recent study of adolescents (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0040963) found increased gains for procedural memories learned directly before the night’s sleep (as opposed to procedures learned 7.5 hours before sleep). But for declarative memories (such as facts and knowledge), this same study found better retrieval among those learning in the afternoon compared to in the evening, right before sleep. But two other less-recent studies (http://www.ncbi.nlm.nih.gov/pubmed/16741280; http://www.ncbi.nlm.nih.gov/pubmed/18391183) got different results–that declarative memories were most effectively consolidated when acquired directly prior to sleep, as opposed to after an 11-15 hour period of wakefulness. So the jury is still out…

As for sleep intervals, it does appear that even shorter sleep periods, such as daytime naps, do facilitate learning and memory consolidation. However, the research on how naps compare to a full night’s sleep is equivocal and may depend on the type of learning (i.e. procedural, perceptual, declarative, etc.). Some studies suggest that napping improves declarative memory retention but not procedural (http://www.ncbi.nlm.nih.gov/pubmed/16647282); other studies suggest napping improves procedural but not declarative memory (http://www.ncbi.nlm.nih.gov/pubmed/16931152). Again, the jury is out on this one too!

The one consensus that seems to run through most of the literature is that slow-wave sleep is a crucial component of the memory consolidation process.

Thank you for reading!

Rick October 4, 2012 at 9:05 pm

I’m impressed with the detail of your comments, but please don’t bury such great info down here! I’m hoping you’ll revisit the general topic of sleep at some stage in a new post.

One other thing about sleep. I notice that doctors and scientists follow the general public in talking casually about “getting” sleep, as if it were something we could get easily, just like going to the store to “get” some milk. I find that, if I go to bed early, I just stay awake longer before falling asleep; if I’m really exhausted and do fall asleep quickly, I wake correspondingly early, and can’t go back to sleep even if I’m still exhausted and would really like to sleep a couple more hours. Given this situation, the obvious and easy answer would seem to be medication, but I’ve also seen mention of a study that claimed that people who take sleep medicine die earlier than those who don’t.

Lindsay Miller October 4, 2012 at 9:43 pm

Very interesting point about the language we use to talk about sleep–I never even thought about that! And I often have the same problem: even when I have enough time to go to bed early or “on time,” I find myself unable to fall asleep. You have certainly given me some food for thought, and I hope to write about sleep again with some of these questions in mind.

Thank you for your feedback.

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