I sleep with my left hand tucked under my head. My hand goes numb during the night. If I keep it at my side it does not. It also goes numb when I am holding a phone to my ear. Also, I have three trigger fingers when I wake in the morning. That stops late
Nerve compression. These symptoms sound a lot like typical carpal tunnel syndrome which often occurs at night and with certain activity like holding a phone or driving. An evaluation from a hand surgeon can easily determine if this is suspected and confirm with ultrasound or nerve studies. A high percentage of patients have trigger finger as well if they have carpal tunnel syndrome.
Don't Panic! Usually numbness induced by certai postures is caused by some sort of "compression" on the nerve supplying that area, in your case probably is in the neck area. Change your posture in sleeping, do gentle neck stretching exercises daily. If no improvement in few weeks you must see a doctor.