It is common to wake up and cry, and it is normal to wake up and laugh.
I woke up laughing in my dreams twice myself. Looking back, it turned out that I hit back when I was watching a comedy during the day.
Because most of the emotions in dreams come from experiences during the day, if you encounter sad things during the day, or have seen tragedies, you may cry in your dreams at night.
And if you encounter happy things or funny things during the day, you can laugh at night, just as the saying goes, you will laugh in your dreams.
So, don't believe that sentence