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
Sleep-wake cycle if external cue is removed will be which of the following? 
Select and explain. 
A. Unchanged, B. Greater than 24 hrs, C. Less than 24 hrs, D. Greater than 12 hours
B: Greater than 24 hrs
Answer: B: Greater than 24 hrs 
Explanation: Circadian clocks keep running even when the environmental cues (changes in light and dark) are removed, but the period of this free-running rhythm is generally a little less or a little more than 24 hours. More frequent is more than 24 hours. Ref: Chest Medicine: Essentials of Pulmonary and Critical Care Medicine edited by Ronald B. George, Richard W. Light, Michael A. Matthay, Richard A. Matthay, 2010, Page 461. 2. Psychiatric Mental Health Nursing By B. T. Basavanthappa, 2007, Page 141