The only way to truly and directly measure a person’s level of fatigue is to get right in there and see what is going on in their brain. One way of doing this is through looking at brainwave activity. In a research setting, this isn’t too hard to do. We stick a bunch of electrodes on your head and we measure the electrical activity and display it as a pattern of brainwaves. In an operational setting, this really isn’t feasible yet….but I am sure a portable and easy to use brainwave measuring system isn’t too far in the future.
A much easier way to determine a person’s level of fatigue is to measure it indirectly through performance on a psychomotor vigilance task (PVT). PVT results are strongly correlated to fatigue risk factors such as acute and chronic sleep disruption, continuous wakefulness and circadian rhythm disruption. This means that if you don’t perform well on a PVT, it could be due to fatigue caused by one or a combination of these risk factors. The PVT has long been the gold standard for estimating fatigue and its use today can be traced back to the pioneering efforts of Dr. Dinges .
The Best On-Line PVT Example, provided by The Sleep Disorder Center Florida: http://www.sleepdisordersflorida.com/pvt1.html
There are many variants to the PVT, but in its most simple form, a person would watch a display and when a light came on, the person would have to press a button to turn off the light. The light turn would turn on randomly every 5 to 10 seconds and the person would continue the task for up to ten minutes. The old systems would monitor how many times you experienced a lapse in attention and missed responding to the light.
Lapses in Attention, impulsivity, reaction time and accuracy on a PVT all get worse when you become fatigued.
The newer systems are much more computer driven and can display a flash of light like the old systems or a variety of different stimuli to be able to monitor other aspects of performance. For example, one variant displays a coloured shape, like a red circle, and then 5 to 10 seconds later, another coloured shape like a red square. Your task would be to respond as quickly as possible and indicate whether the second shape was the same or different than the first shape. This type of system monitors lapses in attention when you don’t respond with a short time frame, impulsivity when you respond without the second shape being displayed, reaction time when you do respond and accuracy of your “same/different” response. All of these measures worsen with increasing levels of fatigue.
Want to see if your PVT performance says you are fatigued? Try this great example of a PVT provided by the American Academy of Sleep Medicine accredited Sleep Disorders Center Florida: http://sleepdisordersflorida.com/pvt1.html.
 See for example: Dinges, D. & Powell, J. (1985). Microcomputer analysis of performance on a portable, simple visual RT task sustained operations. Behavioral Research Methods, Instrumentation, and Computers, 17, 652-655.