What hormone's buildup increases the pressure to sleep through the homeostatic sleep drive?

Adenosine plays a crucial role in promoting sleep by building up in the brain over the course of the day. This accumulation creates a homeostatic sleep drive, which is the body's primary mechanism for regulating the need for sleep. As adenosine levels rise, they contribute to increasing sleep pressure, making the individual feel sleepier. This effect is particularly important because it helps the body stay in sync with its natural circadian rhythms and dictates when an individual feels the urge to sleep.

In contrast, serotonin and melatonin are hormones related to mood and circadian rhythms, respectively, but they do not directly cause the buildup of sleep pressure in the same way that adenosine does. Leptin, while involved in regulating energy balance and appetite, does not have a direct influence on sleep pressure. Therefore, the role of adenosine as a sleep regulator makes it the correct answer in the context of homeostatic sleep drive.