There is a common belief that the average human only uses a small percentage of his or her brain. The common number associated with this is 10%. This leads us to believe that incredibly smart people like Albert Einstein are as smart as they are because they are able to somehow tap into the rest of their brain's potential and utilize it unlike the common person. Modern research has shown this to be false. Scientists can actually monitor brain activity and see not only how the brain acts under different circumstances, but what parts of the brain are being used as well. In reality, everyone uses all parts of their brain at different times for different tasks. While it is true that lying down to take a nap or watch TV utilizes only a small portion of your brain, the reality is that basic daily human activity utilizes 100% of the human brain.
Also, the whole left side versus right side of the brain is not really useful here. While the brain is divided into different parts and these different parts do perform different functions, it is not accurate to talk about only using the left or right side of the brain because in reality both sides and all parts of the brain are constantly used in conjunction with one another to perform daily tasks.
So we use 100% of our brain, on both sides, all parts.