Our body’s alarm clock is extremely good that it is able to know when we need to be up. Our bodies know its daily regular schedule—what time we usually go to sleep, what time we wake up, and even what time we get off the bus/train. To attempt to explain the mechanisms behind this, some scientists refer to the studies of honey bees. Honey bees use their internal body clocks to mark daily events. Bees are (obviously) not able to tell time. However, they are extremely prompt in their daily routine of collecting pollen. Their internal clock determines when they will visit a specific type of flower at different times of the day. For example, they will visit one type of flower at exactly 2 pm in the afternoon and then another species of flower at exactly 6 pm. Their body clock tells them specifically what plant species to visit at specific time intervals. It is believed that the human body works similarly.
[wp_ad_camp_2] There may be a second reason to why we often wake up just in time for our train or bus stop. It also may have to do with the stage of sleep we are in when napping on public transportation. The first two stages of sleep are the lightest stages of sleep, called NREM (non-REM) sleep. This is when we are most prone to be woken up. When our commutes are rather short (40 minutes or less), our naps on the bus/train tend to be even shorter, usually 15-30 minutes. In those 15-30 minutes, most of us are still in the initial stages of non-REM sleep—where we are between being awake and being asleep. In this stage, not much is needed to awaken us. When we are in this stage of sleep, our bodies are very responsive to environmental cues—the sudden stops of the bus, the nudge of that person next to us, the familiar sounds of the environment of our bus stops.
The mechanisms of sleep are still largely a mystery for scientists and much discussion continues to try to get to the bottom of why we sleep and why our bodies are so good at regulating our regular schedules. But as far as why we wake up right at our stop goes, the two reasons above seem to be the most reasonable of explanations at this point.