8 Invisible Signs You're Totally Fried

"We often see patients who are healthy yet extremely fatigued telling us that they're starting to forget things they should remember," says Anne Marie Albano, PhD, the director of the Columbia University Clinic for Anxiety and Related Disorders. "I tell them about the shelf theory of memory: You can only put so much on the shelf at one time, and when you're exhausted, the shelf isn't supporting memories the way it should." You are focusing so intensely on the mental challenges at hand that everything else is subconsciously deemed irrelevant. This is due to the stress hormone cortisol's complicated effect on memories, explains John Ratey, MD, in his book Spark: The Evolutionary New Science of Exercise and the Brain. Excess cortisol can cause us to lose the ability to form and store new memories not related to the present situation and can also make it difficult to retrieve the memories we already have.