• Problem: Poor-quality sleep

A person could sleep the recommended amount, but still feel tired because he or she got poor quality sleep.

"Oftentimes, we only think of sleep in terms of minutes -- but that's really the quantity of sleep. In fact, there's a quality of sleep," said Breus, author of the book "Beauty Sleep."

The average sleep cycle takes 90 minutes to complete. It starts from stage 1, the lightest sleep, and progresses to deeper levels through stage 4. Then, it continues to rapid eye movement (REM) sleep when the person dreams.

"When you have poor quality sleep, you spend a lot of time in stage 1 sleep," said Shives, a spokesperson for the American Academy of Sleep Medicine. "You will tend to want to sleep longer to make up for the quality and increasing quantity."

• Problem: Other medical conditions

Too much long sleep on weekends does not seem to make people feel better
--Dr. Daniel Kripke

"If you find you're sleeping a lot -- like more than nine hours on a regular basis, you need to talk to your physician, because that probably means you got poor quality sleep and that could be sleep apnea, narcolepsy or restless legs form of sleep disorder," said Breus.

Sleep apnea is a major cause of problems, when breathing stops during sleep.

"If you have sleep apnea and you stop breathing through the night, you might feel really tired in the morning even though you've gotten eight hours. Those eight hours were horrible, light, crappy sleep," Breus said.

Another possibility is periodic limb movement disorder, which causes a person to jerk and twitch during sleep, causing disruptions. Depression has also been associated with excessive sleep.

• Problems: Genetics

Some people naturally thrive on little sleep. And then, there are others who flourish after nine hours.

"It's only when you go less than six and a half hours of sleep, we call you a short sleeper; more than 9.5 hours, we call you a long sleeper," Shives said. "We think there's something genetic."

A study published this year in the journal Science identified a mutated gene in a mother-daughter pair that allowed them to function on six hours of sleep. The research conducted by University of California, San Francisco scientists is believed to be the first to discover a gene, hDEC2 involved in regulating sleep length. This finding could someday lead to a better understanding of why some people require more sleep.

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