I think it depends on your view of work/life balance. Difficult when you are small, because theres no back up plan, its you or nothing, so the balance tends to be dictated to you
If your clients expect you to be available 24/7, and smiling when you get dragged out of bed on a freezing Sunday morning, thats fine, so long as you feel you are being fairly recompensed for your time.
I try not to work late/weekends because :
1) I am not a drone. I have something that likes to think its a life outside of work, and its nice to spend time there occasionally
2) "All work and no play makes Jack a dull boy". If I convert my desk to a Borg recharging station and live at the office, I think its going to affect the quality of my work, because it affects the quality of my life. If you're not happy, why bother? Retiring at 89 with enough cash to marry Anna Nicole Smith, and no happy memories doesn't fit my criteria of a successful life, frankly
On the other hand, if its a big client, or you feel like doing them a favour (or you have a "double bubble" clause for after-hours service), then fine. Just don't let anyone get the idea that its their RIGHT to get you at the weekend without paying big bucks for the privelege.
On the other hand (thats 3 hands now - I am turning into a mutant. Help!), if your plan is to work like a dog until you're 30, then retire to the Carribean or something, then I suggest you forget how to sleep :)
You've got to pick a balance thats right for you and how you want to work. Doing 100+ hours/week will make you rich, but if you die of a heart attack, its not worth much.
Some people thrive on that sort of pressure, and don't understand why anyone would want to sleep more than 3 hours at a stretch. Find whats right for you, and stick with it. If a client-free weekend leaves you ready to face the rigours the week, that seems like a plan, but also bear in mind that one of your great strengths as a small business is your flexibility.
I often end up working late for an hour or so, doing some weekend work etc, but thats a choice (I also find that its easier to concentrate due to fewer interuptions). And if I don't feel like it, I don't bother. while I want to be successful, I'm not going to feel my life is a failure if I don't end up in the next Sunday Times Rich List