Yeah, that's my experience. For one example: we're not talking about high-order intellectual skills like writing poems, just some simple software programs. I hacked out a bunch of little civil-engineering programs from '90 to '93; then later when I was in this unpleasant long slump in '95 I remember wanting to change them around a little in order to solve this new problem or that, and even though I wrote them myself I couldn't figure out at all any more how half of them worked. I'd look at the source code in utter confusion and say "what's that supposed to do?" It wasn't that the code was especially complex either, because a couple years later when I was in a better state of mind they were all perfectly clear to me. Same thing happened again in '00 and '01.
Books you read easily last year become as though they were written in Greek. You forget a lot of stuff, lose your car keys. It's rough when you're young and it hasn't hit you several times in the past; you think, "oh no, what's happening to me now, I'm becoming permanently mentally retarded!"
This obviously had an impact on my "workplace competence." Probably there was a bigger effect arising from the way that when I'm in a down phase I flinch and grumble whenever a cow-orker comes into my office with another job, both because I'm tired all the time then and also because when I'm depressed the obnoxious bastards piss me off by presuming to breathe in the same room as me. This is effectively counterbalanced by greatly-increased fear over getting fired (imagine having to go out and find a new job in the trough of a depressive slump!). So while I'm way less competent in a low phase I'm also a lot better motivated to work extra-late and grind and grunt the stupid product out anyhow.
Yours WDK - WKiernan at concentric.net