Sjöstedt would complete the rest of his education at Uppsala University, gaining a Bachelor of Arts (Swedish: filosofie kandidat) in 1921, a licentiate in 1927, and becoming a Doctor of Philosophy in 1930.

During the 1930s, Phalén, along with a group of other philosophers including Sjöstedt held private meetings to discuss the place of philosophy as a subject within the education system of Sweden.

In 1939, Sjöstedt acted as headmaster at the Läroverk [sv] (form of Swedish secondary school, similar to a gymnasium) in Borås, Västergötland.

Aside from work in pedagogy and mathematics, Sjöstedt was a supported of the international auxiliary language Interlingue (then known as Occidental), created by the Estonian Edgar de Wahl.

Aside from these, Sjöstedt also published several books about geometry theory and philosophy, including De filosofiska problemens historia (The history of philosophical problems) in 1936, for use in teaching high school philosophy, alongside Översikt av den formella logiken (An overview of formal logic).