Seconded!
If you're working 19 hours a day, there aren't many hours when you're not working.
As such, there's no reason to put too many limits on when your 1st employee can work (so long as those 40 hours/week are met and stuff is getting done).
You may wish to consider allowing telecommuting, as well as to allow employees to save every hour over 40 they work per week as vacation time ("comp-time"?).
Another alternative: as the employee what type of schedule s/he would like to set, and make it a cooperative process rather than an autocratic process. Autonomy is a huge component of job satisfaction (along with complexity and a clear payoff between effort and reward), so the more autonomy you can give, the more likely it is that you'll keep this person happy enough to stay.
The last full-time job I worked, I was the only full-time employee, and was not allowed to set my own schedule, nor to choose my own tasks, and I was frequently required to work 10-20 hours per week of unpaid overtime. Needless to say, my happiness levels jumped significantly when I quit that job, in spite of the tenuous nature of substitute teaching and non-profit work.
Hopefully your first employee will have a much better experience.