As Allie at Hyperbole and a Half says, it is disappointing to feel sad for no reason. But this is the state in which I find myself again, so I might as well write about it.
Possibly the most disappointing aspect of feeling sad for no reason is that one always has memories of the days when one did not feel sad, when one got out of bed wanting to do things, and it is disappointing to feel rising terror every time anyone suggests doing Big things, such as leaving the house.
I have long suspected that I was designed to hibernate for most of January and February. Some year I may try this. I can re-appear in March, with the daffodils and snowdrops and so on.