There is a nightbus that goes between Sihanoukville and Siem Reap.
Name of the bus company is Virak Buntham Express.
Leaves from the departure city at 8pm and arrives to destination around 6am the next morning.
Busses are nice and clean. Complimentary small bottle of water is available.
Busses are almost always full so please plan your trip and book tickets at guesthouse or bus company office.
It stops on the way to destination city in Phnom Pehn. From SHV arrives to Phnom Penh around 12am and from Siem Reap arrives to PP around 2am.
You can use it as a nightbus to PP and the ticket is cheaper.
Normally the ticket costs $19.