if it's drama (by which I'm thinking you mean play? or film? or TV script?) I assume that's all you need and then cut to/dissolve to/lights up stage left (not quite as sure of terminology here--ask a playwright for that

) a portrayal of the nightmare events. You don't need/want more because actors know how to act; they'll get "INT. Bedroom. JOE in bed having nightmare"
If it's fiction, prose, and he's your POV character, you can't describe what he looks like from the outside, as he's asleep and even were he awake (barring a mirrored ceiling), he could not see himself moving or lying there. If you're in omniscient, or if some other character is looking at him, you could. But if some other character is looking at him, you can't see his dream.
ETA: you may already know all this but here's a how-to on script formatting, note the section on "transitional instructions":
http://www.storysense.com/docs/SPFormat.pdf.