That assumes you get to spend all 16 as you choose.
If you have to spend 9-11 hours at work, more time helping with homework and housework, more time shuttling dependents around, etc., etc., you can end up with very little time left to spend how you choose. Adding 4 hours to that could be like multiplying your one free hour that's left by 5.
There have been times in my life where I was able to get about 14 hours of sleep a day. It would still take me a couple of hours to mentally 'spin-up' but once my brain was in gear the focus and acuity were amazingly productive!
There have been other times in my life when I've been forced to live on less than 5 hours a night and I was a complete zombie almost every 'waking' minute.
So, I'm with BillTheCat, I'd love to be able to sleep less but maintain the higher productivity level, that I can currently only get with many hours of sleep.
If you have to spend 9-11 hours at work, more time helping with homework and housework, more time shuttling dependents around, etc., etc., you can end up with very little time left to spend how you choose. Adding 4 hours to that could be like multiplying your one free hour that's left by 5.