Illustration: John William Waterhouse/Getty Images

During a quick nap, you had a crazy dream about your coworkers turning into mice wearing cardigans.

But wait (you're thinking): A short doze means I'm taking care of myself—and don't doctors recommend taking a nap as a way to get a second wind? The problem here isn't with the siesta; it's with what happens during that time period. It usually takes about 90 minutes to enter REM sleep and start dreaming, says W. Christopher Winter, the medical director of the Martha Jefferson Hospital Sleep Medicine Center in Charlottesville, Virginia. The maximum length of your nap should be around 30 minutes, because once you've hit REM, the nap becomes counterproductive (you'll wake up groggy instead of refreshed, says Winter). So if the mouse dressed like your cube-mate scampers out as soon as you close your eyes, it means your brain is so sleep-deprived that it's rushing to get into the REM phase. A scheduled nap can help make up for one late night, says Winter, not a month's worth.

You're convinced that you've got early-onset dementia—at 33.

"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.

Instead of helping you burn off steam, your usual 45-minute jog makes you feel even worse.

If you're really frazzled when you start to exercise, your body reacts differently than if you aren't in such a state, explains Sarah L. Berga, MD, professor of obstetrics and gynecology at Wake Forest School of Medicine in Winston-Salem, North Carolina. Berga and her colleagues have observed that moderately-intense exercise can cause cortisol levels to spike even higher in women who are already so stressed that they've been missing their periods. At the same time, the "hyperstressed" women's glucose levels dropped, which meant there was less available energy to fuel their exertion. Berga suggests taking a break from hard workouts and signing up for a package of Pilates classes instead.

Next: How effective is caffeine for you?