There are people that wake up with swollen body parts and there are a few reasons for it:
Dehydration is caused by lack of water in the body. When this happens the body starts saving the remaining water in the body which causes swelling of the limbs.
Sleeping posture and duration of sleep
Sleeping on your stomach and on your side is not recommended since it may cause joint ache. Insomnia or insufficient sleep may cause swelling.
High level of sodium
If you have excess salt in your body it will detain water.
This organ is responsible for detoxifying the organism and it is not working properly it will detain water.
Swelling can be reduced with warm or cold compress. Use cold compress if you have allergies or internal problem.
This is the best solution for swelling. If the area is in pain use gentle circular motions instead of pressure.
If the swelling happens during the night, wear compressive materials such as socks and tights or gloves.