Before going to work and at the end of the day, I've started tracking my mood. To keep things simple, I limit myself to three options: +1, 0, and -1. This way I can quickly decide how I feel and I'm always calibrated the same way. While work certainly plays a role in shaping my mood, it's not the only factor. Sometimes my mood is shaped more by what's going on at home.
I'm curious whether this kind of data could help managers interact with their teams more effectively. If someone's been feeling great, it's probably worth talking with them, so you can celebrate together. Or, if someone's been going through a rough patch, it may make sense to wait before assigning another stressful project. And lastly, at a team level this could be a useful "real-time" measure of morale.