Complex sentence: Opiates do not directly cause hallucinations of any kind, but large doses may cause the subject to become so relaxed that they experience dreams while partially awake; dreams which can be so vivid as to be confused with reality.
Split into simpler sentences: Opiates do not directly cause hallucinations of any kind, but large doses may cause the subject to become so relaxed that he or she experiences dreams while partially awake. They can be so vivid as to be confused with reality.