Kaida et al (2007) say: increased EEG alpha activity predicts when you'll nod off. (as do self-reported sleepiness and heart rate variability)
Cajochen et al (1995) say: activity in the 6.25-9.0 Hz range (theta/low-alpha) increases as you stay up longer.
Finelli et al (2000) agree; theta activity increases as you stay awake longer, as does delta (slow-wave) in your next sleep. Delta drops off exponentially as you sleep.
So that's if you can measure your alpha activity over time. What if you only have a few minutes? The Alpha Attenuation Task looks to be the trick: measure your EEG alpha with eyes open vs. eyes closed. It looks like EEG alpha with eyes open decreases as you get sleepier, while EEG with eyes closed increases. So if your eyes open / eyes closed ratio is high, you're not very sleepy, and vice versa. The task was developed by Stampi, Stone and Michimori in 1993 (which I can't find) and studied again in 1995. They found it to correlate well with the MSLT. Alloway, Ogilvie, and Shapiro in 1997 found that it distinguishes narcoleptics from "normals". It seems to be another useful measure of "sleepiness" (as possibly distinct from wakefulness).
Åkerstedt and Gillberg (1990) give a pretty good analysis of what happens in the alpha (8-12hz) and theta (4-8hz) bands as you get subjectively sleepier: they increase, but particularly this high-theta-low-alpha range (5-9 hz). More if you're you're sitting still with your eyes open (as opposed to walking around and doing whatever). If you close your eyes while you're sitting, theta shoots way up, and 10-11hz goes down. This jives with the AAT findings: alpha jumps when you close your eyes, but only if you're not sleepy. (their 5-min eyes-closed, 2-min eyes-open EEG task has become known as the Karolinska Drowsiness Test or KDT.)
In validating the Karolinska Sleepiness Scale, Kaida et al (2006) compared it with a bunch of other measures and found it valid. Interestingly, these all showed correlations: the KSS, the VAS (another subjective sleepiness scale), the KDT and AAT (tests of alpha power with eyes open/closed), and the PVT (response-time test). Which means that the AAT/KDT correlate with both subjective scales (KSS/VAS) and response-time tests (PVT).
In summary: as you stay awake longer (and therefore get sleepier), your theta increases, alpha gets less sensitive to you closing your eyes, subjective sleepiness increases, and task performance goes down.
Other side notes:
- you can run a KDT in 7 minutes, and an AAT in 8 (I think I saw that somewhere). Can you maybe run it in 1 minute? 2 minutes? Or maybe you could just test their theta output for 1 minute?
- Or maybe it'd be possible nowadays to have an office worker or someone just keep an EEG by their desk and test themselves (for 8 minutes) every so often.
- I keep trying to integrate all this with my brewing 2-drive idea, where there's one drive that makes you sleepier and one that makes you more awake, and everyone just keeps talking about the sleep drive when the wake drive needs some research too. It's hard to do.