Though you can often find me dismissing claims made in the movie Inception, occasionally, the scriptwriter sprinkles in a bit of truth. The chemist had too much to drink, so it's raining in his dream. While this particular causation is unlikely, it's not unheard of for real-world events to manifest in some form within the dream, including but not limited to bodily conditions.
As a child, before the
mind finishes maturing, one may find oneself dreaming of going to the
bathroom, only to wake and find you've wet yourself. I myself
experienced this particular dream well into my teens, and only in my
late teens did this stop; instead, I found myself waking with an aching
bladder, but no actual accident. The latter still happens to this day.
or radio alarms can manifest inside the dream as well, as nearly
anything, from car horns to actual alarm. Music playing outside the
dream can change the tone or the behavior or the storyline of the dream.
lights, too, can be reflected within the dream, and some devices that
help induce lucidity (help wake your awareness within the dream) use
this property, including Stephen LaBerge's
Kvasar. Even without such a device, you too can train yourself to use
outside stimuli to help you gain lucidity, either through then leaking
into the dream, or through the WBTB technique.