Final answer:
If someone wakes up and recounts a vivid dream, they were most likely in REM sleep, the stage of sleep most associated with dreaming and memory of dreams.
Step-by-step explanation:
If someone wakes up and talks about the vivid dream they just had, they were most likely in REM sleep (Rapid Eye Movement sleep), which is the sleep stage associated with dreaming. While we do have dreams during other stages of sleep, the dreams we remember upon waking are most often from the REM stage. This is because REM sleep features brain activity that is similar to being awake, which aids in remembering dreams. The work of Rosalind Cartwright and other researchers supports the idea that dreams reflect life events and emotions, indicating a link between our daily concerns and the content of dreams during REM sleep. Additionally, research has shown that lucid dreaming, where the dreamer is aware they are dreaming, also primarily occurs during REM sleep.