20% time rule

The Google Story suggests that this practice existed in academia, where it was brought from. But I can't find external references to it easily:

At Google, the preference is for working in small teams of three, with individual employees expected to allot 20 percent of their time to exploring whatever ideas interest them most. The notion of "20 percent time" is borrowed from the academic world, where professors are given one day a week to pursue private interests.