This is similar to when we set alarms to wake us up, sometimes we wake up even before the alarm goes off. In actual fact, you may not be exactly sleeping, you may be waking up in between naps to check if it’s your stop or not, and when it’s not, you go back to sleep. This is especially true when you commute a particular place all the time, your body would already get used to the environment. Which is why you’re likely to wake up before you get to your bus stop.