Severe drowsiness and reduced alertness immediately after waking, known as sleep inertia (SI), are recognized to impair cognitive performance and reaction speed, thereby negatively affecting daily functioning. While recent studies suggest that the characteristics of alarm sounds may help mitigate SI, analyses that incorporate individual differences—such as sleep duration, age, and psychological factors—remain insufficient. This study aimed to investigate how individual factors influence changes in SI caused by different alarm sounds. Data were collected from 41 participants who met the condition of being able to use the iPhone “Clock” app. Participants were categorized into university-aged (18–22) and middle-aged (30–59) groups, and multiple regression analysis was conducted. The level of drowsiness immediately after waking was assessed using the Karolinska Sleepiness Scale (KSS). The results showed that changes in SI induced by different alarm sounds are influenced by multiple interacting factors, with age and the perceived importance of the next day’s schedule emerging as particularly significant. Based on these findings, future work will aim to develop a system that automatically generates optimal alarm sounds tailored to individual sleep patterns and personal characteristics.