We are wanting to get the sleeper bus from Sanya to Nanning. Does it exist?

Yes, there are and the buses leave at 12:30, 13:20 and 14:30 from SY Bus Station.
Two buses arrive at Langdong Bus Station in Nanning and one reach Jiangnan Bus Station.
The bus journey takes about 12 to 13 hours.

Thanks,

We got the 12.30pm bus and didn't arrive until 7.30am (which was fine for us as the local buses were then running but we had booked an extra night as we thought we were getting in at 1 or 2am) not sure if its because we had to wait an hour for ferry or if it was because the driver stopped at 4am until 5am for a sleep.

Hope this helps people in the future