Why would the pressure in my kitchen faucet drop overnight?
You have to work your way back through the plumbing system to track this down, but start with the easy fix of a clogged aerator. Many kitchen faucets have an aerator that can be easily unscrewed and they usually have a mesh that not only aerates the water, but acts as a filter. If there was any junk in the pipes that got to this point, you can just wash it off (just don't drop any parts down the drain).

Beyond that, you are left with checking and potentially replacing each valve from the sink to the next junction that doesn't have any issues.