It is recorded over a 24 hour period, starting and ending at midnight PST. So you will get yesterdays figures at 9am-ish.
There are peaks in traffic throughout the day such as in the morning, after school kicks out and in the evenings, when people most commonly surf the net.
The lowest figures are throughout the night in the US. So, your viewing figures will not go up much between about 5am and 12pm, when most Americans will be in bed. If you have more worldwide traffic, you may not even see much difference in visitor numbers throughout the day.
Does that answer your question?