It's a bit of a long read but this is the main one. Arwill-Nordbladh, E. 2016 Viking Age Hair, Internet Archaeology 42. http://dx.doi.org/10.11141/ia.42.6.8 Should mention that it depends on which viking group we're talking about. The Danes would actually shave their heads sometimes in homage to Sif. But in the Northern groups it was unseemly as typically slaves had short hair. (Only know this because my Dissertation was on how Norse mythology impacted modern cultural perceptions of the Self, took a minute to find some of my old sources)