We all recognize that a person under the influence of alcohol subjectively feels they are functioning at their peak performance, when objectively they demonstrate significant declines in cognitive and motor performance. So, even when sober, how can you determine if you are fit to perform your expected tasks safely and well?
This is not easily done when you are tired unless you have a standard with which to compare yourself. Unfortunately, the areas of the brain most compromised by fatigue are the same areas required to evaluate and recognize the deficits of the fatigued state.
When soldiers who have slept as little as four hours per night for several weeks were questioned about their performance, they indicated they were functioning very well--maybe better than when rested! In actuality, they were functioning at about 30-35% of their rested capacity! Functional losses in fatigue are very similar to those caused by the influence of alcohol.
Unless one uses an objective set of questions to assess their physical state, the subjective evaluation is usually, "OK, I'm tired but my function is not impaired much!" In the world of aviation, most pilots carefully pre-flight their aircraft before flying. However, rarely do they check themselves for appropriate levels of mental and emotional functioning.
Here is a little checklist I developed as an objective check for myself before I do something complex or difficult. Download it below, print it, trim it, then carry it in your pocket for quick reference. It could save your life!
Remember, intelligence, skill and determination are not enough to overcome the deficits of a tired brain. Never make important decisions or operate machinery when not rested!