I am trying to have a part change material during the night. Why isnt the Part changing Material?

This code is in a server script and I want the part to change when It is night time. and metal material when it is day.

while true do
local timeofday = tonumber(game.Lighting.ClockTime)
if timeofday >= 18 or timeofday <= 6 then
		game.Workspace.Part.Material = Enum.Material.Neon
elseif timeofday <= 18 and timeofday >= 6 then
		game.Workspace.Part.Material = Enum.Material.Metal

end
wait()
end