I recognize the thrust of this argument, and anecdotally agree with it from my experience,but it's not practical advice. Work 12 hours, spend at least 30 minutes each way commuting,work out, prepare and eat meals, and you're at pretty nearly 15 hours, without counting preparing for work. Sure,you have time for sleeping 8 hours,but only if you eschew any personal life and find your work sufficiently exciting to not see any need for recreation/fun all week. Dropping to about 6.5 hours of sleep at least doubles time for those things each work day.

I've tried maintaining 8 hours of sleep daily in such conditions. What I've found is that I'm productive for two days. By Tuesday or Wednesday night, I'm frustrated and bored, and spend 3 or 4 hours doing something interesting and, bam, week derailed. For me, 6.5 hours is the compromise that works. Plus, I sleep 9 hours on Friday and Saturday nights.