Woke up with my left arm completely numb, it's happened twice within the same day, I wasn't sleeping on it though. Is it safe to go back to sleep?

Neuropraxia arm. Numbness of an upper extremity during sleep is common. This event is usually from compression of the blood supply to the extremity. Allowing blood back into the arm by changing positions is the treatment. If the numbness continues after being awake for more than 10 minutes, it may be a more serious condition and would involve an examination by your doctor.