Let’s imagine you have a busy day where everything is taking a little longer than planned. Suddenly, it’s dinnertime and your to-do list is only halfway done. You power through to cross off every last thing and somehow still manage to get in bed at a decent hour. Success! But then, lying in the dark, you can’t sleep even though you’re totally exhausted.

Sound familiar? Harris says this is a prime example of brain fatigue, which happens when we don’t give our minds ample time to wind down and recover before bed. “When you’re going, going, going, it’s hard for you to turn your brain off,” she tells mbg, making it difficult to fall asleep and/or stay asleep through the night.