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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