Dreaming is a natural part of sleeping. Every time we sleep, we dream, even though sometimes we may not remember what that dream was about or that we dreamt at all. This raises the question what the purpose of dreaming is and why we dream at all. There are many theories on this topic giving different purposes, such as learning reinforcement or rehearsing threat perception. After all, it cannot be said, which theory will prove as right, thus leaving the question why we dream unanswered.
http://www.scientificamerican.com/article.cfm?id=tetris-dreams (last access 09.06.2013)
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