"Light" (or at least, lighter) sessions aren't really something I do on purpose, at least consciously, but do happen and serve an important purpose from time to time. It tends to happen when I start off by talking about something less central to the "big" issues we're focused on, or perhaps something I feel strongly positively about, or even just something I feel more distant from than the more heavy, intense, emotionally fraught topics that usually come up. If I were consciously feeling like things had been too much recently I'd probably just say that though, and see where things went from there.