Clearly there seems to be a case for it if HITRANS, Caledonian Sleeper and Scotrail are looking at the possibility of it.
Firstly, I don't think that CS or Scotrail are looking at it. HITRANS have mooted it, and CS said they would look into it if asked. But they will already know the answer.
Secondly, it doesn't follow that because a local authority suggests something, that there is a clear case for it. Far from it. I imagine that this will have been the idea of one individual who doesn't understand the costs of operating a railway, and has not thought about the potential alternatives.
The reality is that north of Dingwall the line is used by fewer than 200,000 people a year, and around half of those are from stations south of Tain inclusive, most of which will be trips to Inverness. most of the rest is from Wick and Thurso.
I'd be surprised if more than 20,000 trips were to or from the central belt or south thereof, and a good proportion of those will be on rover tickets. I'd be surprised if income for the whole line was more than £2m pa, and very likely to be less.
The current CS sleeper operation costs about £50m a year. A Central belt to Far North sleeper would be in the region of £5m-£10m pa.
Even if, optimistically, the sleeper attracted 50 people a night, and they all paid £100 for their tickets, revenue would be £1.5m pa. Realistically patronage would be about a fifth of that, and average ticket prices around half.
And yet you can fly from Wick to Edinburgh in an hour for £130.