Technically: No! We use the email address as the user name for logging into the app to make things easier for the employee and to enable self-service functions such as initializing the start password or resetting the password. Of course, this also makes things easier for the IT administrators.
If the administrators are prepared to manually create users directly in the clock:it go backend via the web interface or manage passwords for the users, then the email address is not mandatory and the user name can be freely chosen. However, as the number of employees increases, the administrative workload also rises quickly.