Why do we dream?

Have you ever stopped to wonder why we dream at night? From sweet dreams to recurring nightmares, our mind is rarely silent – regardless of whether we can really remember their content in the morning.

Sometimes, we find our dreams are linked to things going on in our lives right now, worries about future events or strong memories from the past. This therefore seems to suggest that dreams are in some way linked to our memory, but exactly how, no one seemed sure.

Recent research has investigated the role of dreams and REM sleep (the phase of deep sleep) in the consolidation of long term memory. Consolidation just means the process whereby our memories move from short term to long term storage. In our long term memory, memories are stored for recall. Rehearsal (thinking about) these long term memories briefly involves short term processing, and this rehearsal strengthens the storage of these memories. Dreams may play a part in this consolidation and rehearsal process.

To find out more about REM sleep and our sleep cycle then why not read my previous post here.

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Photo by clownbusiness/Shutterstock, with additional illustration by Lisa Larson-Walker

As I mentioned early, our dreams can have similarities to events which have taken place in our lives. Some research has focused on investigating the content of our dreams and found that the events which tend to be included in our dreams are ones which are rated as more emotional, although not more stressful, than those not incorporated (Malinoski & Horton, 2014). This suggests that REM sleep might help to process emotional memories. Further evidence to support this hypothesis is that levels of REM sleep are lower in people with depression (Cartwright, 1983) and PTSD (Ross et al, 1989).

However, although these dreams can contain elements of real life, they are often distorted: it is rare for the complete memory to be ‘played out’ in our dream. It is been suggested that this is because during sleep we cannot access full episodic memories (memories of events) but instead just traces of them.  This has been hypothesised to be due to reduced hippocampus (the part of our brain involved in memory processing) activity during REM sleep (Buzsàki, 1996). The fact that our dreams can contain strange events or impossibilities is thought to be due to a lack of activity in the prefrontal cortex – the area involved in attention and logic (Stickgold et al, 2001).

In addition to consolidating episodic memories another proposed function of our dreams is to enhance learning of procedural tasks (Smith et al, 1996). Studies in rats have found increased levels of REM sleep after procedural learning, and that less REM sleep resulted in poorer memory (Smith et al, 1985).

Whilst REM sleep and our dreams may be useful for certain types of memory consolidation, it doesn’t mean that this is the only way consolidation takes place, or that it is needed to consolidate every type of memory (Stickgold et al, 2001). The authors of this review hypothesize that dreaming enables the brain “to identify and evaluate novel cortical associations in the light of emotions… during REM”. To put it simply, when we dream our brain is working on processing new memories, learning procedures, and our emotions to events.

 

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The psychology of sleep

Hi everyone, this week’s post follows on from last week’s, which was all about our body clock (if you’d like to read it then click here). First, I will look at the stages of sleep and the link between sleep and memory, before moving on to talk about a sleep disorder – narcolepsy.

Scientists have been able to identify the different stages of sleep by using brain imaging methods such as EEG. This measures brain potentials from the scalp to establish brain activity. Five stages of sleep have been identified – these are shown in the sleep cycle diagram shown below.

sleep-cycle

I’m sure you’ll have heard of the 5th stage – Rapid Eye Movement, or REM sleep. During this stage of sleep, all motor movement is stopped to prevent the body from acting out our dreams.

REM sleep has also been linked to memory consolidation involving our hippocampus. This is a structure in the centre of our brain which has been found to be important for long term memory. It is thought that when we are asleep, the hippocampus is involved in replaying previously activated cells, which establishes our long term memories.

Tilley & Empson (1978) tested the hypothesis that REM sleep is involved in memory consolidation by testing participants’ recall of a story after deprived stage 4 sleep, or deprived REM sleep. They found that participants who had disrupted REM sleep had significantly poorer recall than participants who had disrupted stage 4 sleep.

The involvement of sleep in helping learning has also been shown by neuroimaging studies. Maquet et al (2003) measured patterns of brain activity of participants while they practiced a reaction time motor task. They then measured their brain activity while they were asleep, and found the same patterns of activity appeared during REM sleep. Interestingly, the amount of reactivation of these patterns correlated with the extent of learning, which suggests sleep is vital for consolidating our memories and aiding learning.

Narcolepsy:

This is a sleep disorder which affects 0.5-1% of the population, and is characterised by excessive sleepiness. Patients’ sleep cycles are disrupted, and they have fast entry into REM sleep (Vogel, 1976). Sudden sleepiness is usually brought on by exciting or emotionally charged events, and is often accompanied by a loss of muscle tone, called cataplexy. It is thought to be an intrusion of REM sleep into wakefulness, which is shown by one of the symptoms – sleep paralysis. Narcolepsy  can be caused by a lack of a chemical called orexin, which regulates sleep (NHS Choices). This reduction in orexin has been hypothesised to be caused by cells in the immune system mistakenly attacking the cells that produce orexin, however this has not been proven.

I leave you with the video our lecturer showed us when we learnt about this topic – a dog with narcolepsy. Enjoy – 

I hope you liked this post and don’t forget to leave any requests for future posts in the comments!

The science behind our body clock

Hi everyone, this week’s post is about something we talk about a lot – our body clock. Even though everyone’s heard of it, I’m guessing it’s not something people are too familiar with, so I’ll try and explain it here.

The 24 hour cycle we live in accordance with is known as a Circadian Rhythm. This is set according to our environments – in particular in response to the amount of light or darkness. Therefore, the eyes are really important in regulating our circadian rhythms – people tend to forget that our eyes have functions other than sight!

The signals from the retina on the back of the eyeball travel towards the brain via ganglion cells – specialised cells which carry signals containing the visual output. These ganglion cells project towards a structure called the Suprachiasmatic Nucleus (which is fortunately abbreviated to SCN!) in the hypothalmus, which is shown in the image below:

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The SCN is known as the body’s ‘master clock’, and is responsible for keeping us in time with the circadian rhythm. Studies have shown that if this structure is damaged, then people’s circadian rhythms are virtually abolished. Cohen & Albers (1991) carried out a case study of a 34 year old woman who had part of her hypothalamus taken out as part of an operation to remove a brain tumour. Although the surgery successfully removed the tumour, she was left with a condition called hypersomnolence, which is basically excessive sleepiness in which patients can fall asleep at any time, although the extra sleep does not help reduce the symptoms. This shows the importance of the SCN in regulating our sleep pattern, and how disorientated we would be without it.

I hope you found this post interesting – check back next Thursday for my next blog post all about sleep.