Final answer:
The roommate experiencing a vivid dream about being chased by an insect was most likely in REM sleep, which is known for brain activity similar to wakefulness and vivid dreaming.
Step-by-step explanation:
Your roommate who reports dreaming about a large insect chasing her around your dorm room was most likely in REM sleep. REM, or rapid eye movement sleep, is characterized by brain waves very similar to those during wakefulness and is typically the sleep phase where dreams are most vivid. During REM sleep, the eyes move rapidly under closed eyelids, which is a distinct feature of this phase of sleep. Moreover, REM sleep can sometimes be associated with REM sleep behavior disorder (RBD), where the muscle paralysis that normally accompanies REM sleep is absent, potentially resulting in physical activity during sleep, especially when experiencing disturbing dreams.