Definition
This idiom is used to warn someone that they will become uninteresting if they work all the time and never take time to relax or enjoy themselves.
He's been working non-stop lately, with no time to relax. I think he needs to remember the saying 'all work and no play makes Jack a dull boy.'
I'm worried you're working so late all the time; you'll become very dull. You need to rest more, remember 'all work and no play makes Jack a dull boy'.
The company has been so busy with the big project, I think employees need more time to relax. They need to keep in mind that 'all work and no play makes Jack a dull boy'.