Feeling down in the dumps every so often is a normal part of life. But when you're gripped by an unrelenting sadness or hopelessness that keeps you from going about your usual routine, it's time to pay attention: it's the hallmark sign of clinical depression, and an estimated 7% of adults will experience it, according to the National Institute of Mental Health. Even with this telltale sign in place, it's tough for a depressed person to know if she really has the disease.

"Almost all of the symptoms of depression on their own are experienced by everyone at one time or another," explains Jennifer Payne, MD, director of the Women's Mood Disorders Center at Johns Hopkins University in Baltimore. If you've been dealing with four or more of the following symptoms every day for two weeks, and they've impaired the way you usually function (for example, prevented you from working, being a responsible parent, or seeing friends), it's time to check in with your doctor.

Depression leaves you withdrawn and checked out, and that can manifest as a loss of appetite. "If your brain is preoccupied with negative thoughts, you may forget to eat or lose interest in cooking or preparing meals," says Yvonne Thomas, PhD, Los Angeles-based psychologist specializing in depression and self-esteem. On the other hand, sometimes the disease kicks in the opposite effect, making you hungry and driving you to overeat. "The mix of emotions that tend to depression-sadness, pessimism about the future, and low self-esteem-can compel you to try to soothe your feelings with food binges," says Thomas.

You're sleeping too much or too little

Some people with depression find themselves snoozing under the covers more; the disengagement and dip in energy make you tired all the time, says Thomas. "Sleeping more is also a way depressed people escape from their sadness; it becomes a refuge," she adds. Others with depression experience restless or interrupted sleep or even insomnia.