There is a sentence:
The study has also found that being sad for ten percent of the time is actually good for you. Let's face it. We can't be happy all the time.
I don't understand the meaning of "for ten percent of the time".
I wonder if it refers to once out of ten times. That is, it's good to be sad once in a while.
I'm not sure if my understanding is correct or not. Could you explain this, please?
This sentence is from BBC Learning English.