An item from my search-engine slush-pile “Is ME/CFS worse in winter?”
In my experience, no. My level of sustained physical mediocrity is pretty much the same summer or winter, and any time in between.
Other people may have different experiences, of course – one of the biggest problems with getting ME/CFS taken seriously is that the symptomology is almost infinitely variable.
However, feeling worse in winter may be nothing whatsoever to do with ME/CFS and everything to do with the fact that it’s winter. I know perfectly healthy people who feel crappy in winter, it’s a reaction to the shorter days, darkness, and colder rain than we get in summer (a small climate-change joke there, folks, at no extra charge).
Then there’s winter induced depression, ranging from a vague feeling that all is not well – winter blues in varying degrees – to full-blown and disabling SAD.
The bottom line is that winter can make anybody feel crappy – you don’t need ME/CFS at all for that.