Identifying the symptoms early can help you or a loved one get treatment.

Doctors diagnose mental illnesses by looking at symptoms. The right diagnosis is critical because it points you to the right treatment, but psychiatric diagnoses can be tricky.

Symptoms of different mental disorders–like major depressive disorder, bipolar disorder, and postpartum depression, to name a few–often overlap, and disorders frequently occur together.

“Seeing a pure syndrome–in which the patient only shows the symptoms of one disorder without any symptoms of another–is pretty rare,” says Charles Nemeroff, MD, PhD, chairman of the University of Miami Health System’s department of psychiatry and behavioral sciences. One of the best examples of this, he says, is the overlap of anxiety disorders and depression. “Most people who are depressed are also anxious,” he says.

Research is now showing how much these conditions share in terms of genetics too, which means another aspect of pinpointing a mental health diagnosis is understanding a person’s family history. “These disorders are largely genetically driven, so they run in families, and that’s important to know,” Dr. Nemeroff says.

Having just one symptom on this list doesn’t necessarily mean you have a mental illness. There’s almost always more than one sign, and a psychiatrist making a diagnosis will take all of them into account. “We look at each of the symptoms and [how] they hang together,” Dr. Nemeroff says.

Only a trained specialist can make a definitive diagnosis, and different mental illnesses are characterized by different symptoms. But there are certain common warning signs to look out for in general. Typically, you’re looking for several of these symptoms that occur together–and for changes in behavior.

Withdrawal and apathy

Retreating into yourself and avoiding hanging out with friends or participating in social activities could be a sign of depression, bipolar disorder, schizophrenia, anxiety disorders, and more mental health concerns.

“The [person] is not being as interactive, maybe staying at home in their room more,” says Anita Everett, MD, president of the American Psychiatric Association.

That’s not to say that taking a little “me time” is a bad thing. Spending time alone can be an act of self care, but a loss of interest in the activities you once enjoyed could be a warning sign. As with most symptoms of mental illness, this isolation often represents a departure from past behavior, Dr. Everett says.

Emotional outbursts

At the opposite end of the spectrum from the quiet of withdrawal and apathy are what Dr. Everett calls “loud” features of mental illness. “These can be more in your face,” she says. “People are more irritable, more animated.”

Someone in the manic phase of bipolar disorder might be super-gregarious, talking very rapidly, or spending lots of money. A person with schizophrenia might be visibly distressed if they think they are being followed or spied on.

Problems with thinking

In depression, this could mean trouble focusing and remembering things. You may feel muddled trying to coordinate routine tasks, or you may feel more indecisive than usual.

Similar problems with memory and concentration can occur with schizophrenia, along with difficulty solving problems and slower reaction times.

Other mental illnesses have similar and distinct cognitive problems and, not surprisingly, these issues can affect day-to-day functioning. This could be pronounced, such as problems at school or work, or more subtle, like difficulty navigating bills and appointments.

Cognitive problems can lead to some of the changes that may signal a mental illness: absenteeism from school, dropping grades, missing work. “All of these dramatic changes are warning signs,” Dr. Nemeroff says.

Trouble sleeping

Up to 80% of people with a mental illness have problems with sleep. It’s especially notable in different anxiety disorders, depression, attention deficit hyperactivity disorder (ADHD), and bipolar disorder.

It could mean sleeping too much or too little, tossing and turning, or waking up a lot during the night–and the disturbances can affect treatment.

There’s also a chicken-or-egg aspect: Sleep disorders sometimes up your risk of developing a mental illness.

Everyone can benefit from a good night’s sleep. For most people, that means seven to nine hours a night. Even if you have a mental health condition, the same tricks that work for others might work for you: cutting down on caffeine, exercising (though not right before bed), going to bed and waking up at the same time every day, and not taking your phone or laptop to bed with you. Therapy and medications can also help.

Appetite changes

Not wanting to eat or refusing to eat entirely are the central symptoms of anorexia nervosa, a serious psychiatric disorder. Anorexia often occurs alongside other mental illnesses, including depression and anxiety disorders. Binge-eating disorder and night eating have been linked to schizophrenia.

Eating less or a loss of appetite or interest in food can also be a sign of depression, anxiety, or even everyday stress. Of course, not eating enough can lead to weight loss, and, when dramatic, can be a visible indicator that something is wrong.

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Unusual behavior

One of the main warning signs of a mental illness is change, Dr. Everett says. Maybe a chatty person clams up, or an introverted person becomes oddly outgoing.

“Often these kinds of illnesses don’t happen one day to the next,” she explains. “They happen over time, but the time can be something as short as a couple of weeks or a month. We compare the way things are now to the person’s history.”

While half of all mental illnesses appear by the age of 14, according to the American Psychiatric Association, it may be easier to identify such changes in adults, she adds. Three-quarters of mental illnesses appear by the time a person is 24.

If you notice several of these warning signs in yourself or a loved one, consider bringing up the symptoms with a mental health professional.