As an instructor I do not want to fly with students who are not feeling well. My experience is that the lesson is usually a waste of time for both of us. For personal flying I don't go up in my airplane if the weather is nasty or I am not feeling 100 % as it is supposed to be fun.
For work flying I have a job to do so it is a bit of a different story.
Before every training flight I explicitly ask the student 3 questions
1) Are you in all respects good to go ?
2) Is the aircraft in all respects good to go ?
3) Is the weather suitable for our flight ?