Have you ever noticed that your dreams contain memories? Some scientists suggest that dreams may actually be indicative of the memory replay that takes place when we’re asleep and is important for memory consolidation. They hypothesise that only some elements of a replayed memory, the tip of the iceberg, make it into consciousness and these are manifest as dreams.
Evidence supporting the link between dreams and memory includes the observation that people tend to improve at new skills more if they dream about them. Dr Erin Wamsley, Associate Professor Bob Stickgold and colleagues at Harvard Medical School found that undergraduate students who had dreamt of a computer game maze they had earlier been challenged to learn, improved more than those who had slept but didn’t dream of the game.
The memories we experience in dreams are usually fragmentary – a face, a place, an image. Only rarely do dreams replay a complete scenario that was experienced. This could be because only a small part of the memory being replayed reaches awareness, but it could also be because specific aspects of sleep physiology prevent the various elements of a memory from being bound together. The most coherent memories occur during the more mundane dreams of non-REM sleep early in the night – it was this sleep that the Harvard students were experiencing. Memories that happen during the vivid dreams that characterise REM sleep, a phase that increases through the night, are much more fragmented.
There’s a possible reason for these increasingly fragmented memories. A steroid called cortisol builds up gradually across the night. When the levels have become high during the early morning REM sleep, it disrupts communication between the neocortex, which stores individual memory fragments, and the hippocampus, which helps to bind these together to form complete memories. This also explains why our morning dreams can often be bizarre.