This calls for EXPERIMENTS! Especially since we have this handy Wait function to speed up time.
EXPERIMENT IN PROGRESS...
Shook cancels experiment: interrupted by Bogeymen
Ok hold on a bit, i'll have to do something about those little blighters. So far though, everything up until
V Drowsy only carries a speed penalty.
Edit: OK, now i get a purple/red flashing "Sleep!" icon on the GUI (rather ominous!), and Slumberous on the health screen. This is after staying awake for like 5 days straight, maybe more. Speed is down by like 20%, but i suspect that more sleep dep will be fatal.
Secondary edit: I'm starting to doubt whether any further consequences are to be had. My dorf has been awake for nearly two weeks now, and the SLEEP! marker is still there. Maybe it's just Toady urging you to stay in character? :U
TERTIARY EDIT: It also seems to carry a significant debuff to your effective skill. With a grand master axedorf, i'm having trouble hitting run-of-the-mill bandits. These guys are bad enough that i could dodge their attacks with no dodging skill whatsoever, but it's as if my own to-hit rolls are like 10 times lower. Then a hammerman bashed my face in. But yeah, i don't think there's anything past Slumberous. Be sure to sleep it away though, you can't fight for shit in that state.