In the latest of his excellent columns for Scientific American psychologist Jesse Berring reviews the current theories that try and explain why we’ve evolved to have dreams.
One of the most interesting is the ‘Threat Simulation Theory’ which argues dreams are a form of night-time survival training, based on research that found that dreams often put us in scenarios of personal danger:
In a 2006 study published in the journal Consciousness and Cognition, Zadra, Desjardins, and Marcotte performed a content analysis on a set of 212 recurrent dreams reported by participants ranging from 18-81 years of age.
Among their findings, escape and pursuit themes were the most frequent type of threat found in their sample (25.9 percent), followed by accidents and misfortunes (19.7 percent), aggression and violence (19.0 percent), physical difficulties (17.0 percent), emotional difficulties (7.5 percent), and disasters (3.4 percent).
Furthermore, in nearly all cases the dreamer him- or herself (rather than a stranger or loved one) was the specific target of the threat and usually the dreamers actively participated in some way to resolve, escape, or combat the threat.
The article covers a whole stack of alternatives and is written in Berrings’ usual engaging style.
Link to ‘Dreaming of Nonsense: The Evolutionary Enigma of Dream Content’.
Mind Hacks 
What evidence is the evidence that cannabis can increase the chance of psychosis? Can you provide citations?
Our world is much more complicated than the world our ancestors lived in. I wonder if the bizarre and nonsensical contents of our dreams isn’t the result of an insanely huge number of various building blocks falling into the mechanism that evolved to construct possible scenarios from just a few.