I think people often find that with self-employment they encounter a far more demanding boss. "Keeping it for yourself" I think implies a certain amount of lack of pressure or freedom, and while that can be truthy ... it's not how it feels.

Self-employment / being an entrepreneur is a complex situation.

Self-employed involved another 8 hours a day of work to keep your pipeline full. Plus you always have some ‘customer’ whether it is a consulting client or a web site visitor or a search engine or advertiser, etc.

It’s quite a philosophical question. Working eight hours a day for someone else can, in a way, be seen as outsourcing your own will: you think, decide, and act—but always within the framework of what someone else needs. Self-employment, on the other hand, can be deeply rewarding, but it also requires a great deal of self-discipline. In my view, that’s where you run the risk of becoming your own worst boss: demanding, inflexible, and with no clear boundaries.

Is it actually possible to rent out your brain for _only_ eight hours a day? Or does stuff inevitably spill over into the other 16?

More like selling your soul for 35 years.