Found this helpful insight here.
The trend in industrialized nations over the last hundred years suggests that people are increasingly sleep deprived. In 1910, the average person slept 9.0 hours a night. By 1975, the total had fallen to 7.5 hours. The 2002 Sleep in America poll, conducted by the NSF, indicates that the average American adult now only sleeps 6.9 hours a night, leading to fatigue, exhaustion and other symptoms.
I have been averaging about 5 or 6 hours of sleep a night during the week. Maybe that is why I have been feeling so out of sorts lately.