What external factor primarily affects the SCN and its function?

The suprachiasmatic nucleus (SCN) is a critical part of the brain that functions as the master circadian clock, regulating various physiological processes according to the day-night cycle. One of the primary external factors that affects the SCN and its function is light.

Light, particularly blue light, has a significant influence on the SCN. Specialized retinal cells, known as intrinsically photosensitive retinal ganglion cells (ipRGCs), send signals from the eyes to the SCN in response to light, helping to synchronize the internal body clock with the external environment. This process is essential for maintaining circadian rhythms, impacting sleep-wake cycles and various hormonal and metabolic functions.

While factors such as sound, temperature, and nutrition can have some effects on sleep and circadian rhythms, they do not primarily regulate the SCN's function in the same way that light does. Light exposure, especially in the morning, is key to resetting the SCN and aligning its activities with the environmental light-dark cycle. Thus, the direct and profound impact of light on the SCN underscores its role as the primary external factor affecting this nucleus and overall circadian regulation.