Yeah I get that quite a bit these days as well. For me it's because my sleep schedule is so out of whack that I'm almost always sleep deprived, to the point that I can't focus on the things that I _want_ to get done, and just amble around doing little of anything productive even if I have energy until I eventually decide it's too late and call it a day.
In general, how many hours of sleep do you average? I used to get 6.5 hours and I'd feel meh but able to power through things with naps in the evening, but just last week or so it's been more like 5 or less and I've found I can't function effectively at that point. Worse still, that my sleep gets delayed due to said naps in the evening, further perpetuating the cycle.
I'm trying to break out of it and thus far it seems to be getting slightly better (although sleep debt can take a loooooong time to recover from - something like 4 hours of extra catchup sleep required for each hour lost in the nights before) so there's still hope if this is the case in your situation.
Gimmick
What is your daily schedule like? I think that's where an analysis can begin first
edit: apparently the comment box ate some of my text
JennyJackal
Basically: Wake up at 7am, go to work half an hour later, work until 5 or 6pm, come home and crash.