Short-acting Ambien-type drugs are supposed to have less respiratory-depression (not a good thing at high altitude) than Valium-class benzodiazopines.
The highly respected Institute for Altitude Medicine of Peter Hackett at Telluride specifically says:
For other sleep disturbances at altitude: Ambien (zolpidem) 5-10 mg by mouth before bedtime as needed for sleep
Ambien CR (controlled release) 12.5 mg by mouth before bedtime as needed for sleep
Lunesta (eszopiclone) 1-3 mg by mouth before bedtime as needed for sleep Inst Alt Med
