Hamsters run on a wheel constantly through out the night and even day depending on the time the hamster is asleep and awake. Since this is natural for a hamster, you will need the appropriate sized wheel. If the wheel is to small the hamsters back will arch and will most likely will cause spinal problems. Your hamsters back should be completely straight. Even if their back arches only about a centimeter, over time  it could become an issue.

The line shows how the hamsters back and neck doesn't arch. This is what you should see when your  hamster is running on their wheel.

Good Wheel Choices 

Bad Wheel Choices

The line shows how the hamsters back and neck is arched. This is very unhealthy and is very painful to the hamster. This is what your hamsters back and neck should NOT look like when their running on their wheel.

A hamsters wheel cannot be wire or mesh. If the hamsters wheel is a wire or mesh wheel they will most likely get an infection called "bumble foot." Bumble foot is when the hamsters paw(s) develop sores and bumbles. They will usually start out as a small red bump and will then overgrow. It infects the hamsters paw(s) from small cuts or scrapes then bacteria are able to enter the hamsters foot pad. If left untreated it will overgrow and become very painful to the hamster. This can happen for many reasons but it can be caused by a wire or mesh wheel. Other ways the paw can be infected with bumbles foot is, obesity, wire bottom cages, poor sanitation, and over grown toenails.