It can be quite stressful. In contrary to what many people think professors* do not just teach. They are indeed teachers and they should do this job well but this is only part of their working time (in the UK 40% of their contract time generally). They should also produce excellent research (meaning NEW knowledge not in the literature) so uncertainty and hard work fills their days because you do not know where research will take you and if you are going to have a paper worthwhile publishing to a good journal. If you are lucky you have built a research group but this of course means you should also be a good manager and a good mentor to your graduate students.

Seeking funding: On your research output you mainly base your applications for funding which take up a lot of time but have very low chances of success as this is a very competitive game. For example, all my August will be mainly spent writing an EU proposal with a very uncertain outcome. Add to these the several meetings and travels to talk about your research and discuss it at conferences and in other universities and elsewhere -quite a bit of preparation is needed generally for those presentations.

Admin: departmental duties (admit undergraduate and postgraduate students, participate in examination committees and do other tasks.) sometimes, you organize seminar/conferences/workshops which again takes up a lot of time.

adding all this together sounds like no free time exists. However, I believe relaxing is essential, preferably by doing sports (although I do not exactly follow what I advocate and I usually watch a TV series -although I do walk to my work and back.)

Overall, that an academic cannot work less than 50 hours per week - many times more.

*=permanent faculty member.