I feel like this a lot too... I do think for me it is on days when I am in a constant struggle to keep myself above water. Some days I give in to it and I'm in bed all day and some days I manage to keep myself afloat but many days I'm right on the edge and I go in and out just like you describe... Crying jags, bits of productivity, short spurts of focus. Today has been like that... I was on a hotline earlier then suddenly felt better and ended up taking a nap. Then amazingly actually got a bit of work done this evening.