Like most facets of society still lingering from the industrial revolution, the 8-hour work day (or 5-day work) week makes no sense. Why? The 8-hour work day causes deadweight loss.
Deadweight loss is "a loss of economic efficiency that can occur when equilibrium for a good or service is not achieved or is not achievable." Basically, it's when neither party involved in a transaction is able to maximize their own value.
So why does the 8-hour work day cause deadweight loss? Because it implies that you come to work for 8 hours and then you leave. While you're at work you work. When you're 8-hour sentence is up you stop working. These boundaries work great for a factory worker. But it makes no sense at all if you're in a knowledge-based job.
Let's say your commute takes an hour if you get to work at 9am but it takes 30 minutes when you get into work at 10am. The time difference is deadweight loss. Your employer doesn't get any extra work out of you and you don't get to sleep in an extra 30 minutes.
Or what if you have to pick up your bike from the bike mechanic but your work day + commute overlaps perfectly with the bike shop's hours. This means you have to pick up your bike on Saturday or Sunday. When everybody else is also at the bike shop. The time you spend waiting in line to pick up your bike is deadweight loss.
Or what if it's an amazing day outside? If you're tied to your desk during an arbitrary range of time during the day then you don't get to go outside and play and your employer is paying you to tweet about how much you wish you were on your bike. Again, deadweight loss.
Two years ago I quit my 9-5 and started to use my time efficiently. Trust me, don't put your happiness in the hands of a 20th century industrialist.