You tuck yourself under the covers, turn out the light, and look forward to eight hours of blissful slumber. But, after turning for hours you're still exhausted, and no closer to sleep than when you first got into bed. Let's talk today about sleep disorders.

Sleep disorders fall into four basic categories. The scenario I described, in which you toss and turn because you can't fall asleep, is called insomnia. Another type of insomnia is when you wake up in the middle of the night and can't get back to sleep. Sometimes people get insomnia for a night or two because they're stressed out over a big meeting at work, or they're excited about an upcoming trip. Others can't sleep night after night, and that's called chronic insomnia.

People with the second category of sleep disorders have a hard time staying awake during the day, even if they slept well the night before. This is called hypersomnia. Sometimes doctors can't find a cause for hypersomnia. But in many cases, a health condition like fibromyalgia, a thyroid problem, a disease like mononucleosis, obesity, or obstructive sleep apnea, can make you sleepy. If you notice a co-worker is nodding off in the middle of meetings, he might have narcolepsy, a sleep disorder that causes people to sleep uncontrollably at inappropriate times during the day. Narcolepsy isn't only embarrassing, it can be dangerous if you nod off behind the wheel of a car.

A sleep rhythm problem means that you can't stick to a normal sleep schedule. Maybe you work the night shift at your job, or you're always traveling to different time zones and are constantly battling jet lag. Well, whatever the cause, the lack of a normal sleep pattern is called a sleep rhythm disorder.

The good news is that you don't have to live on fewer hours of sleep, because there are decent treatments for sleep disorders. If you're struggling to sleep throughout the night, and dragging through the day as a result, talk to your doctor, who can refer you to a sleep specialist for an evaluation.