The conduction of sound takes place in two ways: through air and via bone. Our skull also picks up sonic vibrations. This is called bone conduction. Via bone conduction our eardrum vibrates, because we perceive sound; we can hear it.
Therefore, sound can never be blocked out completely, by blocking the ear canal. In case you would like the highest possible noise reduction, we would advise you to use a combination of earplugs for sleeping, as well as a white noise sound machine.