So envy that you can decide to sleep or not to sleep on the plane.... For me, I couldn't get any sleep on the plane at all 
. Anyway, I would suggest you rest as much as possible on the plane.
Depends on where you are going to stay, you should arrive at the hotel at 12am. Hong Kong is a late city especially in party areas like LKF or Wanchai on Fri and Sat night. You may grab a beer if you are not too tired.
Mind you, Hong Kong is very hot and humid now. Over 30 degrees and may reach 37 in midday. You may prefer to see the outdoor sightseeing spots in the early morning and after 6pm.