What decides how deep someone's sleep would be?A small study, published in 2010 in Current Biology, suggests that differences in how sleeping people respond to noise may be related to levels of brain activity called sleep spindles. The researchers found that people whose brains produced the most of these high-frequency sleep spindles were more likely to sleep through loud noises. But more research is needed to confirm the results
It's hard to generalize about what makes some people light sleepers and others heavy sleepers. It might be some sort of genetics, or it might be that some people have a greater degree of arousal over a 24-hour cycle.
In most cases, however, factors under your own control affect the quality of sleep you get. There are lots of issues related to lifestyle, medication, alcohol, and caffeine that can lighten sleep. People might also not be getting enough sleep because they're not spending enough time in bed due to the choices they make.
Practicing healthy sleep habits - maintaining a regular sleep schedule; limiting caffeine and alcohol use; and sleeping in a quiet, dark, and cool space - can all help foster deeper, heavier sleep.
Having said all this, being a heavy sleeper or a light sleeper has nothing to do with the quality of your sleep. Hope this helps.