My forearms go numb from the elbow down when I sit for short times and while im sleep no matter what position what's the cause?

Pinched nerve. This is typically from a pinched nerve...The question is where it is pinched. Typically this occurs in the elbow, shoulder, or neck. By observing the pattern of numbness, the exact nerve and location can often be identified. At other times nerve testing can be helpful.
? cubital tunne. The most common cause of numbness with elbow positioning is "cubital tunnel syndrome" exacerbated by pressure on the back inside of the elbow or prolonged flexion. Avoiding flexion, particularly at night will help. If frequent or constant you should see a neurologist or hand surgeon.