Final answer:
Serotonin levels are low during REM sleep, which is marked by high brain activity but muscle paralysis, suggesting reduced serotonin activity during this phase of sleep as compared to during wakefulness or physical activity.
Step-by-step explanation:
5-HT (Serotonin) levels are known to vary across different states of consciousness and bodily engagement. The question is asking, "When are 5-HT (Serotonin) levels LOW?" The correct answer is B) During REM sleep. During REM sleep, although the brain is very active and dreaming occurs, muscle paralysis happens excluding muscles necessary for circulation and respiration.
This stage is similar to an awake state in terms of brain activity, yet different because of the lack of muscle tone. Since REM sleep is a period where there is high brain activity but not during voluntary motor activities, this suggests that serotonin levels are not at their highest, as they are more associated with periods of activity and wakefulness.
On the other hand, serotonin is implicated in the regulation of mood, appetite, and sleep, among other functions, and levels can be affected by various factors, including sensory input and physical activity.