Is it true that we only use 1% of our brain?

I think the data suggests that we only know about 10% of how the brain functions. The brain is such a complex organ it is yet to be fully understood. Everytime something new is learned about it, it is also discovered that there is even more to learn. I don't think the functions of the brain will ever be completely understood. At least not in this lifetime or in my children's lifetime either. It is a complex series of electrical outputs with so much to learn about it.

Depends what you mean. Our brains are the CPU of our bodies and we use them a great deal. Every single bodily function requires an input from the brain pretty much. We don't notice most of them, breathing, blinking, walking, talking etc, but the brain has a finger in them all. Like most important controls it's important that we don't let them overheat so they have an in built mechanism that allows itself to pretty much disengage whilst it resets itself - typically this happens whilst we sleep. There are aspects of our brains that we may under utilise and there's an awful lot that we don't understand - but we use more brain than 1% for certain.