The suprachiasmatic nucleus or the brain's "pacemaker", is influenced by activity, light, and which one of the following neurotransmitters?

A. Norepinephrine: Norepinephrine primarily regulates arousal, alertness, and attention through its effects on the sympathetic nervous system. Although it plays a role in wakefulness, it is not the key neurotransmitter directly influencing the circadian rhythm of the suprachiasmatic nucleus B. Acetylcholine: Acetylcholine is involved in learning, memory, and REM sleep modulation. It contributes to sleep-wake transitions but has limited direct influence on the SCN’s regulation of circadian rhythms. C. Melatonin: Melatonin is a hormone secreted by the pineal gland that signals darkness and promotes sleep. It acts on receptors in the SCN but is not a neurotransmitter; rather, it modulates circadian rhythm indirectly through hormonal feedback. D. Serotonin: Serotonin strongly influences the SCN by helping synchronize circadian rhythms with environmental light-dark cycles. It modulates the SCN’s response to light input, affecting both sleep-wake regulation and mood-related circadian adjustments.