We are in the midst of a global energy crisis but it has nothing to do with oil. The problem is unexplained fatigue. 

"I'm so tired; I just can't do what I used to do.�?/P>

"I'd love to go but honestly, I just don't have the energy."

"Sex? You mean right now?"

If you're like most women, these phrases have become mantras, the echo of our collective yawn growing louder every day. 

"The single biggest complaint I hear from my patients, day in and day out, is fatigue," says cardiologist Nieca Goldberg, MD, Director of the NYU Medical Center Women's Heart Program and associate professor at the NYU School of Medicine.

Of course, for some of us the problem is simply multi-tasking to the max and not getting enough sleep, or good quality sleep. "If you're continually logging in just 5 or 6 hours a night, it's going to catch up with you, no matter your age," says Rebecca Amaru, MD, clinical instructor of obstetrics and gynecology at the Mount Sinai Medical Center in New York City. 

But if you are getting a healthy 7 to 8 hours a night and you're still tired, Goldberg says it's time for a check–up to uncover the causes for fatigue.

"If your fatigue goes on for more than a week and there is no explanation for feeling tired, then yes, see your doctor," says Goldberg.

While occasionally fatigue may be a sign of a serious illness, experts say most often it's caused by a minor problem, with a relatively easy fix.

To help you zero in on why you can't stop yawning, here are 7 hidden causes of fatigue -- potential health problems you should discuss with your doctor.

Fatigue Cause # 1: Anemia

"If you are in your reproductive years, and particularly if you experience heavy menstrual cycles, have fibroid tumors or uterine polyps, or if you've recently given birth, the blood loss may have caused you to develop anemia -- a leading cause of fatigue in women," says Amaru. 

To confirm a diagnosis of anemia, your physician will give you a blood test. Treatment, she says, usually consists of iron supplements if iron deficiency is the cause, and adding iron-rich foods -- such as spinach, broccoli, and red meat -- to your diet.

The good news: With effective treatment, your fatigue should begin to lift in thirty days or less.