Diffrient's three-volume reference work, Humanscale, explores the relationship of spine to chair and other "human engineering" data necessary for highly specialized workplaces such as a cockpit or a truck cab, as well as aiding in the search for the perfectly comfortable place to sit down.

In the field of furniture design, most notably ergonomic seating, Niels won a total of 24 awards, including two Best of Show and 10 Gold and Top awards. Diffrient holds more than 46 design and utility patents on furniture designs in America and abroad, and his many honors include the I.D. (magazine) Top 40 Design Innovators of 1996, the 1996 Chrysler Award for Innovation, the Smithsonian’s 2002 Cooper-Hewitt National Design Award for Product Design, and the 2005 Legend Award from Contract magazine.