I realize this is orthogonal to your point, but when it comes to knowledge work, especially startup work, I'm highly skeptical of any prescriptive approach to hours worked.

Optimal hours is of scientific interest, but I think someone in a startup even thinking about this is a bad signal. Instead ultimate productivity is more closely related to how focused they are when they are actually working. It's easy to spend 16 hours a day in the office and fritter it away with a million small but unimportant tasks. Likewise, if you come in and get in the zone for 6 solid hours, you might be able to create something of much larger value than anything the multitasker achieved in a frenzied and overworked 80-hour week.

Of course the holy grail is combining a high degree of focus with extended hours, but you can't manufacture this. This is probably the best argument for hiring young idealistic people over seasoned veterans it that they are more likely to naturally put in a lot more hours. However, even unspoken peer pressure to stay in the office is going to be counterproductive, ideally you want people to work as long as they are productive and then unplug completely to recharge quickly. If you get it right and create the right kind of excitement and intrinsic motivation then you can get large gains of subconscious problem-solving as well.