Okay, so I need to figure out the correct answer for this question about the sleep-wake cycle when external cues are removed. Let's start by recalling what I know about circadian rhythms. The sleep-wake cycle is regulated by the body's internal clock, which is primarily located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Normally, external cues like light and darkness help synchronize this clock with the 24-hour day. These external cues are called zeitgebers.

If external cues are removed, like in a study where someone is kept in a dark room without any time signals, the body's circadian rhythm will still persist but might not follow the exact 24-hour cycle. I remember that in such conditions, people tend to have a slightly longer cycle, maybe around 25 hours. This is known as the free-running rhythm. So, the question is probably testing knowledge about what happens when zeitgebers are absent.

The options might include terms like free-running, entrained, or circadian rhythm. The correct answer would be free-running because without external cues, the internal clock runs freely without being entrained to the 24-hour day. The other options might be things like entrained (which is when external cues are present), or maybe something about the specific length of the cycle.

Wait, let me think again. Free-running is when the internal clock runs without external synchronization. Entrained is when it's synchronized with external cues. So if external cues are removed, the rhythm becomes free-running. The other options might be distractors like "entrained," "circadian," or "desynchronized." Also, the term "circadian" refers to the approximate 24-hour cycle, but when it's free-running, it's not entrained. So the answer should be free-running. The clinical pearl here is that the SCN's intrinsic period is slightly longer than 24 hours, so in the absence of zeitgebers, people would experience longer cycles. That's a key point for exams.

**Core Concept** The sleep-wake cycle is governed by the circadian rhythm, regulated by the **suprachiasmatic nucleus (SCN)** in the hypothalamus. In the absence of external zeitgebers (e.g., light/dark cycles), the intrinsic circadian period typically **free-runs** and extends to approximately 24-25 hours due to the SCN’s endogenous pacemaker.

**Why the Correct Answer is Right** When external cues are removed (e.g., isolation studies in constant darkness), the circadian rhythm is no longer entrained to the 24-hour day. The SCN’s intrinsic oscillation, driven by **clock genes** like *Per* and *Cry*, produces a **free-running rhythm** with a period of ~24.2–24.5 hours in humans. This demonstrates the endogenous nature of circadian regulation.

**Why Each Wrong Option is Incorrect**
**Option A:** *Entrained rhythm* is incorrect because entrainment requires external zeitgebers to synchronize the cycle to 24 hours.
**Option B:** *Circadian rhythm* is a general term for ~24-hour cycles but does not specify the free-running state without external cues.
**Option D:** *Desynchronized rhythm* refers to fragmented sleep (e.g., in insomnia), not

✓ Correct Answer: B. Greater than 24 hrs