Can be. On an easier to notice (because you're not relying on a specific event or trigger) level, it's lack of interest in things you'd normally like. I'm not the best at the moment, and I *know* going out cycling will help; the fresh air, the vitamin D from the sunshine, the endorphins when you come back in, possibly a beer in the shower if I know I don't have to drive later on. All of it's great. Didn't manage to get out all weekend and then had to force myself out today. Glad I went now, trying to bookmark that feeling for tomorrow.