The central nervous system is responsible for the waking up caused by the ring of an alarm clock. When a person falls asleep, our brain waves become slower and stabilize. However, certain areas of the brain remain active. As per hearing, that certain part of the brain does not “shut down” during sleeping which causes a person to react or even wake up when there’s a noise. This also explains the process of a person being woken up by an alarm.
Stretching and yawning after waking up is called pandiculation which is a phenomenon that helps us regain muscle tones and triggers our central nervous system into action. As for salivation due to the smell of the brewing coffee and rumbling of the stomach, our peripheral nervous system is responsible for this, specifically the autonomic nervous system. Under the autonomic nervous system, salivation falls under parasympathetic which conserves human energy and is responsible for stimulating salivary glands.