You stayed up late binge-watching The Crown. Then you woke up extra early to beat the boss to the office. Some days, there’s no mystery as to why you need an extra shot of espresso (and if you can’t sleep no matter what you try, check out these 10 sleep myths and real solutions for better shuteye.)
But sometimes, the root of your fatigue isn’t so obvious, and everything from a hidden health issue to your gym habits could be to blame. “It’s like asking a pediatrician why a baby is crying; the answer could be any number of things,” says Tanvir Hussain, MD, a preventive cardiologist in Los Angeles.