I always wondered how we feel being tired - whatever the underlying reason is. Where in the brain does it happen, what system does the perception of beeing tired - reporting it to consciousness? Also, it must exist to stop something harmful to happen to the brain or other parts of the body. Something else: Abnormal fatigue happens after brain injuries but also in depression. In brain injury some basic mechanism - like transmitter metabolism my be damaged, and cause the fatigue, but may fatigue also protect the psyche in some way, or may it sometimes give "false readouts"? I'm amazed how little we understand, but even more that nobody seem to ask the question about or investigate the question of the "tiredness-perception-brain-curcuit".