So far, as you've noticed, the Leaf automatically tracks sleep only from 9 pm till 9 am. Therefore, in case you fall asleep before 9 pm (e.g. at 7 pm) the Leaf won't track those two hours of sleep, however, if your sleep starts before 9 am (e.g. 7 am) it will track your sleep outside of the sleep tracking period until you wake up (e.g. even if you wake up at noon or at 2 pm it will show up on the graph).
You can make up for this by adding sleep during the day as naps. Your naps will not show in the sleep graph, however, they will add on to your total sleep time for that day.