Recommendation
You must beat your baseline for each level to unlock the next level. That means you have to repeat each level at least once. However, many people ask whether they should repeat levels more than once to increase their performance (shown in their number of stars). That’s a great idea. We recommend you challenge yourself to achieve the highest number of stars you can! That said, there is also a benefit to training in many different levels.
Why?
The benefit to training many times in the same level is that the brain changes best when it is challenged. Getting to a point where a task is hard for you—where your brain is challenged—is an important aspect of positive brain plasticity. The benefit to training in multiple levels is that every exercise level is a little bit different than the other levels. Each one works out a slightly different set of neurons. In order to exercise as many neurons as you possibly can, you must train in many similar (but not the same) ways.