Why am I feeling tired all the time?

Q. Lately I find myself so tired all the time. I'm in my late 20s. I have a part-time job and go to school at night. I'm definitely busy and more than a little stressed. My doctor says I'm not anemic and my thyroid is fine. Should I be taking vitamins? Sleeping more? What else could the problem be?

A. A lot of illnesses can drain energy from a person, including anemia and thyroid problems. But usually feeling tired all the time is not due to disease; it's due to life. I can easily understand why you might feel tired. You've got a lot on your plate. Several possibilities other than illness could be at play here.

Your hectic work and school schedule would cause anyone a lot of stress. Work and school both challenge you throughout the day and night, often dealing out nasty surprises like being asked to be in two places at the same time.

When you are faced with sudden and unexpected challenges, you're likely to experience what's called the fight-or-flight response. A sudden rush of stress hormones helps your body deliver a surge of energy to deal with challenges. But afterward you feel depleted. Repeatedly running on high-alert eats up your energy supply just as driving 80 mph guzzles your gas.

So how can you counter stress? When you feel stressed, try deep breathing (inhale as you silently count up to four, exhale as you count down to one) or another kind of relaxation. Practice meditation. Or consider behavior therapy, which helps you recognize — and defuse — your response to stress.

Depression and anxiety are two other common causes of feeling tired all the time. I ask my patients about certain “red flag” symptoms. Do you often feel sad, hopeless or worthless? Have you had problems with your appetite (too much or too little)? Are you having sleep problems or trouble concentrating or making decisions? If you have any of these, you could be suffering from depression.

The red flag symptoms for anxiety include many of the depression symptoms, plus restlessness, irritability and muscle tension. If you think you may be suffering from depression or anxiety, it's worth talking with your doctor. Frequent exercise, talk therapy and medication when needed can help ease these problems.

Too much sleep can leave you as tired as too little sleep. Aim for seven to eight hours of sleep at night; any more or any less have been associated with increased health problems.

What you eat and how active you are can have a huge effect on your energy. Regular exercise makes you sleep better, boosts your mood and helps melt stress. Small meals and snacks throughout the day maintain your energy level better than one or two large meals. That's true even if you get the same number of calories. Vitamin deficiency, particularly of vitamins B12 and D, can cause fatigue. Your doctor can test your blood for these deficiencies.

If you still feel worn out after trying some of these suggestions, call your doctor to schedule a more thorough exam. But that probably won't be necessary: Making healthy changes can energize you and ward off a full power outage.

• Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: AskDrK.com.