 Lighting improved, with the recommended levels in 1916 being around twice the level of the 1890s.Nonetheless, skyscrapers still relied primarily on natural sunlight, which required installing large windows and having tall ceilings to allow the sunlight to penetrate the back of the offices; an office deeper than was not considered a practical design.One of the reasons for the increase in the numbers of skyscrapers during the period was the growth in demand for office workers.