awakeImage by Kistaro Windrider via Flickr
Quantity of sleep (of reasonable quality) is the strongest predictor of my performance at work. All other influences are trivial except to the degree that they influence my quantity of decent sleep.
This is a pretty strong thing to say. Stress only matters to the degree it interferes with sleep. How intrinsically boring what I'm working on is has no effect on my ability to focus. Available distractions don't matter. Distractions in the office barely matter. Meetings- well, they're a time-sink, but how much of my day around the meeting they wreck is strongly defined by how much good sleep I've had. Caffeine keeps me from falling asleep in my swivel chair, but its effects on my quality of work last less than two hours and, in excess, are more than ruined by the resulting insomnia that night.
Corollary: When I am sleep-deprived, the absolute most productive thing I can do is take half a day off and sleep. Four hours of work is nothing compared to the productivity I can gain from four hours of sleep when I need it. I can show up for a useless day, or have a very productive half day- and the next several days from there, if I can keep on schedule.
I just need to remember this. I spent large amounts of the weekend asleep. In the hour and a half I've been in the office, I got more done than I did last Thursday and Friday combined.
The project manager for the group is one of those mutants who needs two hours of sleep a night; I'm not!
However every now and again I'll get really tired after a few days of stress or something, and sleep for 8-10 hours one night. Then its back to the 6-7 hour sleeps.