This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Sleep disorders and eating disorders, Subject name: Psychiatry
Feature of narcolepsy 
Select and explain. 
A. Increased sleep time, B. Decreased REM sleep latency, C. Increase in muscle tone, D. None of the above
B: Decreased REM sleep latency
Answer: B: Decreased REM sleep latency 
Explanation: The hallmark of this disorder is decreased REM latency, i.e. decreased latent period before the first REM period occurs. Normal REM latency is 90-100 minutes. In narcolepsy, REM-sleep usually occurs within 10 minutes of the onset of sleep. The common age of onset is 15-25 years, with usually a stable course throughout life. Ref: A Sho Textbook of Psychiatry, Niraj Ahuja, 7th Edition, pg. no. 138