This has been a common question this week and I've seen it mainly caused by a few reasons. For most cases, it's not alarming and has a simple reason behind it.
1) There are several complex nerves that travel from your neck down the arm. When these nerves become irritated overtime, you can get a numbness/tingling sensation. Although you might be feeling something down in the arm, the problem almost always originates from the neck or shoulder.
The hard part is to determine the cause of why these nerves are irritated. It could be that you have a current injury to the neck or shoulder causing additional stress on the nerves. It could also be that you aren’t using a proper pillow and this places extra stress on the nerves overnight.
2) Overnight you may simply be rolling compressing your arm/hand causing reduced blood flow. When there isn't enough blood going to your arm, the nerves cannot function properly and you can also get this same sensation.
Take the time to assess your own situation!
Dr. Calvin Cheung