Sleep or caffeine?

I have no idea what has changed lately but I’ve been practically falling asleep at work for the past few days. My sleeping habits and schedule are the same and I’ve been falling asleep quite easily. But, for whatever odd reason, I’m completely exhausted all day at work to the point where I actually cracked open a caffeinated Diet Coke this afternoon to try to wake up. After six months with virtually no caffeine intake, I’m now completely wired.