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There’s a lot that scientists still don't know about why people dream and where dreams come from.  However, the prevailing theory is that (like skills and habits) and likely serves as a “rehearsal” for various situations and challenges that one faces during the daytime. We also know much — but not all — of what’s going on physiologically during dreams. Most dreaming occurs during REM (rapid eye movement) sleep, which we cycle through periodically during the night. Sleep studies show as they are when we’re awake. Experts believe the brainstem generates REM sleep and the forebrain generates dreams. In fact, if the brainstem is injured, patients dream but don’t go into REM sleep. And if the forebrain is injured, patients go into REM sleep but don’t dream. At the same time, we have far more to learn about we dream. For example, one study suggests that dreams stem more from your imagination (the memories, ) than from perception (the vivid sensory experiences you collect in your forebrain). Experts have also found that dreaming can accompany psychiatric conditions. We do know that people are more likely to have nightmares. These are manifestations of tension for people living with PTSD because they recur around their traumatic experiences.  Of course, people without PTSD also have nightmares, so more research is needed as to the source of these often-upsetting dreams. But why are some dreams so strange? This may have to do with neurotransmitters, or brain chemicals. During REM sleep, some are more pronounced while others are suppressed. Acetylcholine, , is more prominent, as is dopamine, which some researchers link to hallucinations. Dopamine may help give dreams their surreal quality. Meanwhile, REM that usually keep us awake: histamine, serotonin and norepinephrine, also known as noradrenaline. Thus, we’re less conscious of our environment.