Did you know you can mentally train yourself to wake up right before your alarm goes off? According to researchers, this has to do with how our stress hormones adjust at night to the time we think we're going to wake up:
Your body hates your alarm clock. It's jarring. It's stressful. And it ruins all that hard work. It defeats the purpose of gradually waking up. So, to avoid being interrupted, your body does something amazing: It starts increasing PER [the protein that regulates our sleep-wake cycle] and stress hormones earlier in the night. Your body gets a head start so the waking process isn't cut short. It's so precise that your eyelids open minutes—maybe even seconds—before the alarm goes off.
So if you're not waking up before your alarm clock, don't switch it with one that's louder. What you really probably need is a more regular, predictable sleep routine.

[Mental Floss]