if you have an external filter, depending on your tank type, you may be able to raise the water level and reduce splashing noise that way. don't do this if you have an acrylic tank with a water-level sticker on it -- you'll risk busting a seam.
if all you have is an airpump w/ an underground filter, then there is no way to completely reduce noise. you can add an airflow regulator valve to the airline and turn it down every night before bed, but that's pretty much it for an air pump.
if you have an external and an airstone, you could probably get away with turning off the airpump at night (so long as you aren't overstocked or very heavily planted.) otherwise, you might could add the valve mentiones above and just adjust every night before bed.
if you have an internal filter, you're being too sensitive.