Last night, Kayla thought she was awakened by a noise she heard outside of her bedroom door. When she tried to get up to see what was going on, she found she could not move. She tried to scream but could not make a sound. After several minutes, she was finally able to sit up and wondered what had just happened. The best explanation for Kayla's experience, according to sleep researchers, is that she experienced

• A. a drug-induced hallucination.
• B. side effects from increased amounts of REM sleep.
• C. a nightmare during deep sleep.
• D. sleep paralysis during REM sleep.