Translation:Håll dig vaken!
Interesting question. I think I'd actually use the infinitive instead for that – if I wrote a list of things I needed to do, for instance, I'd say hålla mig vaken. Somehow håll mig vaken sounds like a command to someone else.
For me, I would say "Håll dig vaken" and just talk to myself in 2nd person.
"Somna inte!" Should be accepted. I know it's not the exact same meaning, but it's used in the same way. I've barely ever heard "Håll dig vaken!" while I've heard "Somna inte!" tons of times. Feels way more natural, to me at least.
You specified yourself why "somna inte" is not an accepted answer. While it might mean the same thing pragmatically, it does not semantically.
I can definitely see people use it in other forms. "Håll dig vaken till tio" or something similar.