The sleep-wake cycle shifts in which of the following ways with the removal of external cues?
**Core Concept**
The sleep-wake cycle, also known as the circadian rhythm, is a natural process regulated by an internal biological clock that responds to external cues such as light and darkness. The suprachiasmatic nucleus (SCN) in the hypothalamus acts as the master clock, synchronizing the bodily functions with the 24-hour day-night cycle.
**Why the Correct Answer is Right**
When external cues are removed, the sleep-wake cycle shifts to a free-running rhythm, which means it no longer follows the 24-hour day-night cycle. This is because the SCN continues to generate a circadian rhythm, but it becomes desynchronized from the external environment. As a result, the sleep-wake cycle may become longer or shorter than 24 hours, depending on the individual's internal clock.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it implies that the sleep-wake cycle remains unchanged without external cues, which contradicts the concept of a free-running rhythm.
**Option B:** This option is incorrect because it suggests that the sleep-wake cycle becomes completely random without external cues, which is not accurate. While the cycle may shift, it still follows a predictable pattern.
**Option C:** This option is incorrect because it implies that the sleep-wake cycle is completely abolished without external cues, which is not the case. The SCN continues to generate a circadian rhythm, even in the absence of external cues.
**Clinical Pearl / High-Yield Fact**
In situations where external cues are removed, such as in blind individuals or those living in isolation, the sleep-wake cycle may still be regulated by an internal clock, but it may become desynchronized from the external environment. This highlights the importance of the SCN in regulating the circadian rhythm.
**Correct Answer:** C.