18 hours is 10% less light.. you are now providing 10% less photons.. if you can increase light intensity by 10%, you will still be providing the exact same amount of energy per 24 hours period..
check my DLI chart in week 3.. if your light has proper spec sheet, you can determine roughly how much DLI you are providing.. if too much, the 10% loss would be a benefit, not a negative.
this is all relative to context and equipment.. it's not a black and white answer, but most likely.. no, it won't matter much.. you probably won't notice, even if it is a few less photons.
while moving hte light closer may have similar impact, you lose footprint area.. as well as 'side' light -- the stuff bouncing off walls... when i turn my light metere from vertical to horizontal, my intensity of light changes very little... move a l ight too close and the top-down reading is excellent, but turn to side and you see the plant is clearly not as well immersed with light. (top of leaves is all that matters. so from the side isn't so key, but close to it is)
I see a bit of stretch, so if you haven't get the light intensity up a bit.. if you did, ignore me as closer might be bad.