I use sick leave for mental health days. With how much evidence is coming out about how our mental and physical health are connected, and anecdotally I'm more susceptible to colds when mentally fucked. I also have CPTSD and when that flares I can't sleep, and I can't work if I can't sleep.

I suspect most of my own issues will alleviate with removing myself from call centre work, however. But mental health is different for everyone. Some can work through it (I can work through my depression, for example), and some cannot. It affects everyone differently. Plus, Im not in the habit of divulging my medical history to coworkers, so I just say I'm unwell and will not be in, and I'll get a med cert/Stat Dec, and that's that.

Edit to add: I have never "booked" a sick day in advance though. Like, I've never sat at work and thought "I'm gonna take X day off next week". I use annual for that sort of thing, like maybe a day of annual after a busy period, to recharge. But not a sick day in advance.