I tried to google it but everything I could find was 1) lying to others about being fine, and 2) lying about coming down with a flu or being busy to avoid going out with friends.

Mu question is different: how ethical / morally justifiable is calling in sick (like lying about catching a flu/getting a food poisoning) in academic environment? For me it’s two-fold: I’m both a graduate student and a teacher, and sometimes I cancel on my students (not absolutely last-minute though), or on my academic supervisor because I felt so bad I didn’t sleep and often feel like I’m not up for a class / discussing my dissertation.
There’re three things I care about here:
- morale; 
- my reputation;
- my mental well-being.
Each time I make such a decision, I feel like I prioritise my health, and by this, doing right by myself. Also, in the country where I live, my generation (born between 1995-2000s) is probably the first taking mental health seriously. So I choose very carefully who to mention my depression to in academic setting.
On the other hand, I feel like a pathetic chronic liar anyway. And here come my worries about morale and mental well-being. Am I a pathetic liar? Or is it a sign of some other issue with my mental health?
I will be grateful to hear any relevant experience or opinions of yours.
Thank you!