The primary reason hands go numb while sleeping is due to compression of blood vessels or nerves caused by certain sleeping positions. Common positions that can lead to numbness include sleeping on the stomach with hands under the head, sleeping on the side with bent wrists or arms restricting blood flow, or sleeping on the back with the head resting on the arm. This compression restricts blood flow or presses nerves, causing temporary numbness or a "pins and needles" feeling, which typically resolves once the position changes during sleep or upon waking.

Other potential causes include conditions like carpal tunnel syndrome, where inflammation presses on the median nerve in the wrist, leading to numbness and weakness, especially at night. Repetitive movements during the day or underlying nerve damage from various causes such as cervical spine issues, nerve impingement, or other medical conditions can also contribute.

In summary, numb hands during sleep are commonly due to nerve or blood vessel compression from sleeping posture, and if frequent or persistent, other underlying nerve or health issues should be considered and possibly evaluated by a healthcare professional.