  Abnormal working hours can reduce work health, general well-being, and
productivity, independent from a profession. To inform future approaches for
automatic stress and overload detection, this paper establishes empirically
collected measures of the work patterns of software engineers. To this aim, we
perform the first large-scale study of software engineers' working hours by
investigating the time stamps of commit activities of 86 large open source
software projects, both containing hired and volunteer developers. We find that
two thirds of software engineers mainly follow typical office hours,
empirically established to be from 10h to 18h, and do not usually work during
nights and weekends. Large variations between projects and individuals exist.
Surprisingly, we found no support that project maturation would decrease
abnormal working hours. In the Firefox case study, we found that hired
developers work more during office hours while seniority, either in terms of
number of commits or job status, did not impact working hours. We conclude that
the use of working hours or timestamps of work products for stress detection
requires establishing baselines at the level of individuals.
