Needing a simple psychophysical test to generate reference values of alertness, I looked around for existing tests. The most commonly referenced was the Psychomotor Vigilance Task, which has been used in a number of studies of sleepiness and seems to correlate well with subjective scores of sleepiness as well as with EEG measurements. However it is basically just a reaction time task and it is not clear how best to deal with errors of omission or false alarms. It also seems to me to be the kind of thing that wakes you up - exactly the opposite of what you need when testing alertness!
Inspired by an earlier vigilance task based on watching a clock which occasionally skips ahead (the Mackworth Clock), I've instead created an alertness test of my own, based on tracking a moving object on a touch screen. In this test, the object appears to follow a sinusoidal path, but in fact speeds up and slows down at random times so that the subject needs to be vigilant to maintain tracking accuracy. The test is easy to perform and deliberately monotonous, which I hope will allow for a good measure of alertness. The test provides simple metrics in the form of mean lag, overall accuracy, accuracy in the non-standard cycles, and maximum tracking error. Early results show encouraging correlation of tracking accuracy with self-reported sleepiness.
You can try out the alertness test here.