If you mean "have a glass of wine and soak in the hot tub," then yes. You can and should do this. Your subconscious will be working on the business problems anyway.
But you cannot turn off your cell phone. Opportunities/problems will present themselves 24/7 and you should always be ready to receive them. Each is a golden opportunity to learn more about your business.
This doesn't fully answer your question, but according to one study, "inability to stop thinking about work during leisure time" was a significant predictor of mental fatigue. Constant mental fatigue is unpleasant and is not conducive to productivity. If there's a way you can detach while outside work while still caring while you're at work, that might be ideal.
Making a commitment to leaving work at work is how you have more of a life than just work. I'm all for passion, and for a balanced life, but the bottom line is it's up to you to find the right balance at any given time and be able to compartmentalize as needed.
One of the most difficult fallacies in the startup world is that you should work more than 8-10 hours a day. The returns of intellectual and creative work (business, software engineering, graphic design, etc) RAPIDLY DIMINISH as you try to cram more hours in the day.
Based on this, it's much more important to be disciplined and organized with your productive time (8 hours or so of real work) than it is to try and double the amount of time you spend working. This means prioritizing what needs to be done and staying focused/avoiding interrupts.
One useful side effect is giving you the rest of your time for other things that exercise different parts of you - having a social life, working out and exercising, etc.
There will always be business and technology crises in a startup that leave you working around the clock putting out fires, but you need to recognize this is a failure mode, not a way to do business.
While you need balance in your life, I am not sure how your can run your business and be somewhat detached... Since it is your business, no one will do what you neglect to do.
If you manage to find a good manager that knows how to run the business, and would do great on his own, but doesn't feel like he should take the risk and start his own business, then he might be able to let you be more detached, though you should still handle the payroll and make sure that the money doesn't disappear. I know someone that managed to do just that, but it is very very rare, and it also means that the business is not at its full potential.
So while it would be nice to have the perks of owning your business, but without the hard work, it doesn't work that way in the real world.
If you are passionate about what you are doing, then you will be doing what you love, and you can find a way to have some balance, for example, make certain during the day spend time with family, but you may spend time at night working, as well as having done it during the day.
If you are not passionate, then you will have a hard time getting others to be passionate about what you are doing, which may be a problem if you need employees to go the extra mile, or need to approach VCs.
If you want more balance in your life, you may just need to find a regular 40hr/wk job so you can do something that you are not attached to, and can put behind you at the end of the workday.
I have a passion for solving problems, so it doesn't matter what I am doing, but I tend to think about how to solve different things, even if I will never write the app, because I don't have time, I enjoy considering how I would approach it. If I need a break I will go do something physical, I used to do more mountain biking, and that was nice, as I couldn't think about anything else, else I would have risks hurting myself, so I had to focus on the task at hand. I also go to the gym and exercise, as that takes a great deal of concentration to do well, as I push myself. Even when playing with my children I try to share my passion for building with them, as we build with legos or Kinex.
To switch off, I play squash 3 days a week at lunch time.
For an hour and a half, I don't think about home or work.