1. Anemia

The fatigue caused by anemia is the result of a lack of red blood cells, which bring oxygen from your lungs to your tissues and cells. You may feel weak and short of breath. Anemia may be caused by an iron or vitamin deficiency, blood loss, internal bleeding, or a chronic disease such as rheumatoid arthritis, cancer, or kidney failure. Women of childbearing age are especially susceptible to iron-deficiency anemia because of blood loss during menstruation and the body's need for extra iron during pregnancy and breastfeeding.

Fatigue is a major one. Others include extreme weakness, difficulty sleeping, lack of concentration, rapid heartbeat, chest pains, and headache. Simple exercise, such as climbing the stairs or walking short distances, can cause fatigue.

2. Thyroid Disease

When your thyroid hormones are out of whack, even everyday activities will wipe you out. The thyroid gland, about the size of the knot on a man's tie, is found in the front of the neck and produces hormones that control your metabolism. Too much thyroid hormone (hyperthyroidism), and metabolism speeds up. Too little (hypothyroidism), and metabolism slows down.

Hyperthyroidism causes muscle fatigue and weakness, which you may notice first in the thighs. Exercises such as riding a bike or climbing stairs become more difficult. Other symptoms include unexplained weight loss, feeling warm all the time, increased heart rate, shorter and less frequent menstrual flows, and increased thirst. Hyperthyroidism is most commonly diagnosed in women in their 20s and 30s, but it can occur in older women and men too. Hypothyroidism causes fatigue, an inability to concentrate, and muscle soreness, even with minor activity. Other symptoms include weight gain due to water retention, feeling cold all the time (even in warmer weather), heavier and more frequent menstrual flows, and constipation. Hypothyroidism is most common in women over age 50; in fact, as many as 10% of women past 50 will have at least mild hypothyroidism.

3. You skip exercise when you're tired

Skipping your workout to save energy actually works against you. In a University of Georgia study, sedentary but otherwise healthy adults who began exercising lightly three days a week for as little as 20 minutes at a time reported feeling less fatigued and more energized after six weeks. Regular exercise boosts strength and endurance, helps make your cardiovascular system run more efficiently, and delivers oxygen and nutrients to your tissues. 

4. Depression

We don't all experience depression in the same way. But commonly, depression can cause decreased energy, changes in sleeping and eating patterns, problems with memory and concentration, and feelings of hopelessness, worthlessness, and negativity. 

5. You don't drink enough water

Being even slightly dehydrated—as little as 2% of normal fluid loss—takes a toll on energy levels. Dehydration causes a reduction in blood volume, which makes the blood thicker. This requires your heart to pump less efficiently, reducing the speed at which oxygen and nutrients reach your muscles and organs. To calculate your normal fluid needs, take your weight in pounds, divide in half and drink that number of ounces of fluid a day.

Source: Prevention, Skinny Mom