Great question. As mankind evolved we were designed to be active during the day and sleep during the darkness hours. Our bodies rejuvenate during the sleep periods.
So, health wise this is probably the best schedule for most people. But with the advent of businesses that require staying open 24 hours a day, this cannot always be accomplished.
In the medical field there is a need to have people working around the clock. Over the years i have noticed that those that work the night shifts (11:00pm -07:00am) are more laid back in personality, as the stress levels on their jobs are much less.
Our bodies learn to adapt to the schedules we keep. Although there are thoughts that the quality of sleep is better between the hours of 11pm - 6am, there is no proof that sleeping any other times of the day is not as effective in the rejuvenation of our body functions.