Question: Why are we either a 'light' or 'heavy' sleeper?

Sean Murphy answered on 19 Mar 2012:

It turns out that some people’s brains are better than others’ at blocking the constant incoming flow of environmental stimuli during sleep, and in a new study, scientists have identified and measured the process. The researchers measured the patterns generated by the thalamus, a region deep in the brain that processes incoming visual and auditory stimuli. They found that people with more spindles are more likely to be protected from sleep disruption. This is still early days for this sort of research and they still don’t completely understand how this occurs.