Ivar777. Do you land at HKIA at 10PM or is that the time you expect to get to somewhere within HK city? Also, is your flight at 7AM or do you just need to head to the airport by 7AM?
I ask because I think you should probably try the Macau saunas. If you land at 10PM and depart at 7AM, then its probably not enough time but if your situation is as I ask, then you have plenty of time to hit a Macau sauna.
Here is a schedule for the ferries from HKIA to Macau:
http://www.turbojet.com.hk/eng/schedule/airport.html
Also schedule of ferries from HK to Macau:
http://www.turbojet.com.hk/eng/schedule/prd.html
You can either take a ferry from HKIA or from HK to Macau - its only 1 hour. Either way, you would be in Macau no later than midnight I think. You can then hit a sauna for 3-5 hours and get a couple of sessions before heading back to HK or HKIA. Review the ferry schedules to see if its possible.
Good luck.