REM sleep, memory and depression: an interview with Dr Penny Lewis, University of Manchester


Many people know that REM stands for rapid eye movement sleep, but please could you give some more information about what happens during this part of the sleep cycle and how does REM differ from the other stages of sleep?

Usually you don’t go into REM until you’ve passed through all the other stages of sleep. The stage immediately before REM is slow-wave sleep.

If you were to look at the brain during REM, using an EEG, you would see a quite a rapid, oscillating line, which means that lots of different parts of the brain are doing lots of things that are not particularly synchronized. In fact, brain activity in REM sleep looks very similar to brain activity in wake when measured with EEG.

That’s very different to what happens during slow-wave sleep, when there are high amplitude, slow oscillations at about 1 hertz. This happens when brain activity is really synchronized, with neurons all firing together, then pausing before firing together again, about every second. So there’s this big switch when a person goes into the REM stage, you don’t get that synchrony anymore.

Another thing that happens during REM sleep is that you don’t regulate your body temperature anymore, so you could get warm or cold.

Probably the most interesting thing that happens when you are in REM sleep is that all of your bodily muscles are actually paralyzed, except for your eye muscles (which are darting around). So, you can’t actually move when you’re in REM sleep, you remain completely motionless.

Interestingly, when people have an REM sleep disorder, that paralysis fails and they tend to act out their dreams, which can lead to all kinds of injuries.

During REM sleep, you also tend to have your most vivid, emotional dreams, and these tend to be very fragmented dreams compared to the dreams you have in other stages of sleep.

So, dreams occur through all the different stages of sleep?

Yes, dreams occur all the time.

Girl sleeping

How do memories get strengthened whilst we sleep?

There are several different ideas about this. Probably the most well supported at the moment is that memories get replayed during sleep.

When you practice something, you cause a replay in the neural activity associated with it and that replaying strengthens up the representation. We think the same thing happens when you get replays during sleep, it’s just that you’ve not had to practice, it is just happening spontaneously.

In addition, we think the strengthening that occurs when you’re asleep, particularly during slow-wave sleep, is actually stronger than if you were to replay the memory while you were awake.

Is it possible to selectively strengthen certain memories?

We can selectively trigger replay of the memories that we want to strengthen. This is quite an exciting technique that’s been developed over the last couple of years.

To do this one needs to pair a smell or a sound with the memories experienced during the day, i.e. by having the smell or sound there when a person learns something. This smell or sound then needs to be given to them again during slow-wave sleep.

Because the smell or sound is associated with the memory, it’s thought to trigger the neural activity of that memory, leading to the strengthening effect.

For example, if you learned two lists of words, say words that were read aloud, and then you heard one list read aloud to you again while you were asleep, you would have a better memory of the list that was read to you in you sleep if you were tested on both lists the next day.

We think that is because neural replay has been triggered while you sleep. This is actually a very old idea. However, the current work on this involves several really high profile scientific papers showing that this is really true, and the work that has done also allows us to understand why. We know that it’s about triggering this neural activity and strengthening up representations while you sleep.

It is has been proposed that people who are depressed may be having too much REM. Please could you explain the reasoning behind this theory?

Well, it’s not a theory - it’s an empirical observation that many people who are depressed have excessive amounts of REM, we know that.

We also know that many anti-depressants actually suppress REM and some completely supress it so that you don’t get any REM at all.

The part that is hypothetical is whether suppressing REM could actually help to reduce depression or make people feel better We do not know the answer to this as the experiments haven’t been done, but there is data showing that REM is associated with excessive strengthening of emotional memories and those are usually negative.

It might be that if you‘ve got lots of REM, it may be strengthening up those negative memories too much and that’s contributing to making people depressed and keeping them depressed. Perhaps by reducing REM one can inhibit this process and that might help people to gradually start feeling better.

Has this theory been tested and what research still needs to be done to confirm or discount this theory?

Well, it hasn’t been tested so it’s still a hypothesis at this stage, but what we know is that people with depression have more REM; we know that anti-depressants in general reduce REM; we know that emotional memories and usually negative memories are strengthened by REM… but no one has looked at the interaction between those things. Some people are currently doing research on this.

You mentioned that some antidepressants reduce the amount of REM sleep people have. How easy is this to do and could it have negative side effects?

It is very easy to reduce the amount of REM sleep people have, however, people often worry about whether this has any negative side effects.

If REM is doing something important, then what happens if you totally prevent people from having REM? The answer is we don’t really know.

There are some people that have no REM at all because they’re on anti-depressants or because of brain injury and they don’t seem to have too many negative side effects.

This is something that needs to be studied more though, because if REM wasn’t doing anything then why would we have it?

How much do we currently know about what sleep is really for? How do you plan to find out more?

Well, that’s a broad question! We know that sleep does strengthen memories, but that’s probably not its only function.

In terms of memories, there is evidence that sleep helps to integrate memories together and to extract out the commonalities between many different situations.

For example, what would you expect to find in a restaurant? What would happen there? To answer that question you might extract out commonalities from a whole bunch of different memories of visiting restaurants that were all quite different. We think sleep might be important for extracting this kind of information.

Beyond that, sleep is probably also important for a number of things that have nothing to do with memory. A paper that came out in SCIENCE recently talked about “cleaning up” the brain. Basically, cerebrospinal fluid passes through the brain tissue during sleep, which might allow the cleaning out of toxins that have built up

It’s very unlikely that sleep has one function, it has a number of different functions, but the one that I am particularly concerned with is memory.

Do you think it will ever be possible to cut out sleep altogether?

I don’t think so. I think that whatever sleep is doing, it is essential. The only way that we might achieve something like that is by stimulating the brain to have synthetic sleep, but then I don’t know if you could call that cutting out sleep.

However, it might be possible to make the sleep that we do have much more efficient, so that we get the same benefit from less of it. For instance, at the moment we know that you can enhance slow-wave sleep either with electrical stimulation or with auditory stimulation at the right frequency. That really boosts the amplitudes of slow oscillations and makes sleep more effective for consolidating memory.

Some groups are looking at whether we can do the same things for REM type activities; although I think it’s going to be a bit harder. There is some neuro-feedback work showing that people can learn to trigger oscillations that normally happened during sleep intentionally, when they’re awake, and this seems to improve subsequent sleep.

Where can readers find more information?

Readers can find more information in my book:

The Secret World of Sleep: The Surprising Science of the Mind at Rest published by Palgrave Macmillan -

People with insomnia can find further information on:

About Dr Penny Lewis

Penny Lewis BIG IMAGEPenelope A. Lewis is a neuroscientist at the University of Manchester, where she runs the Sleep and Memory Lab. She has written for a number of popular science publications, including New Scientist.Her research has been featured on the BBC, and she's received funding from top institutes, includingthe Wellcome Trust and Unilever. She lives in Manchester, United Kingdom.


  1. Jackie Small Brown Jackie Small Brown United States says:

    On a typical night, I get less than 30 minutes of deep sleep. I get approximately 2 to 3 hours of light sleep. I tend to get 3 to 4 hours of REM sleep every night.... sometimes, more than that .

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News-Medical.Net.
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