Typically not one specific activity, but a slew of different activities all pointed towards the goal.

An example:

8AM - wake, prepare for day, take bus to campus

9AM - teach class

10:30 AM - office hours, grade, read relevant papers

12:00 PM - short lunch window

12:15 PM - research, write code, notes. Short intermittent breaks. Library visits. Seminar attendance / prep, seminar/journal reading, prep, etc. Consult with advisor, others

7:00 PM - break for dinner, bus home

8:00 PM - research, write code, notes. Short intermittent breaks. Seminary reading, journal reading, etc. Class prep.

3:00 AM - sleep

Weekends were similar to weekdays without class or bussing.