This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Without external cue, the sleep-wake cycle in humans 
Select and explain. 
A. Unchanged, B. Continues with cycle length of > 24 hours, C. Continues with cycle length of < 24 hours, D. Continue with cycle length of > 12 hours
B: Continues with cycle length of > 24 hours
Answer: B: Continues with cycle length of > 24 hours 
Explanation: Normal sleep-wake cycle in humans- 24 hrs
When external cues (Light and darkness) removed, sleep-wake cycle- Slightly increased (>24hrs)