Millions of people struggle to stay alert at home, in school, on the job, and on the road. According to the National Sleep Foundation, 63 percent of American adults do not get the eight hours of sleep recommended by this organization. Moreover, estimates indicate that fatigue contributes to more than 100,000 car accidents that cause 71,000 injuries and 1,500 deaths each year in the United States alone.

To make sure that you don't doze off while reading this, you might take the Daytime Sleepiness Test.

For a sleep overview, visit The Normal Sleep Cycle or Brain Basics: Understanding Sleep from the National Institute of Neurological Disorders and Stroke. Now, if you're feeling well rested or have a steaming cup of coffee to help you along, read on and unravel your nighttime needs.

Many Web sites cover general and detailed information on sleep. Sleepnet.com, for example, describes itself as "Everything you wanted to know about sleep but were too tired to ask." Also, the sleep disorders page of MEDLINEplus serves as a great resource for consumers with questions. Furthermore, SleepDisorders.com and the SleepChannel deliver many resources about sleep disorders for families, patients, and professionals. Anyone who suspects that they might suffer from a sleeping disorder can take the Sleep Disorder Quiz.

About half of Americans suffer from frequent bouts of insomnia, probably the most common sleeping disorder. This condition makes it difficult to fall asleep or can cause a person to awaken frequently during the night. Other insomniacs wake up very early and cannot get back to sleep. You can learn more at the Sleep/Insomnia Program, which also provides an annotated book list, curious cures, and more.

According to the Overview of Obstructive Sleep Apnea from SleepQuest, people who suffer from this disorder experience "recurrent episodes during sleep when their throat closes and they cannot suck air into their lungs." Moreover, sleep apnea can cause death due to heart failure, cardiac arrhythmia, stroke, or hypertension. The University of Pennsylvania Health System page on Obstructive Sleep Apnea shows what happens to airways during apnea and provides many details about this condition.

Jerome M. Siegel's article on narcolepsy in Scientific American states that this disorder affects as many as one in 600 people in Japan, with apparently lower rates in other parts of the world. A person suffering from this disease can unexpectedly go entirely limp and fall to the floor, unable to move for minutes, although remaining entirely alert and conscious. This disease can also cause a person to fall asleep in the middle of a conversation. For more information, visit the Narcolepsy Network or the Stanford Center for Narcolepsy at the Stanford School of Medicine.

A particularly irritating sleep disorder called Restless Leg Syndrome creates an uncomfortable sensation in a person's thighs, calves, and even feet just before they fall asleep. Similar sensations wake the person during the night. The Restless Legs Syndrome Information Page from the National Institute of Neurological Disorders and Stroke covers prognosis, research, and treatment. The Restless Legs Syndrome Foundation supplies a newsletter, research information, and contact details for support groups.

Experts list nearly 100 other sleep disorders, some of which can frighten even the bravest among us. For example, the Night Terror Resource Center describes symptoms that range from seeing spiders and snakes in your sleep to screaming yourself awake. Horror fans, especially ones who like scaring themselves as much as possible but haven't experienced a certifiable night terror yet, will be relieved to know that night terrors can crop up at any age, from six months to a century old. Those who escape nighttime fear might face other twists and turns under the covers. For instance, some people sleepwalk, others sleep talk, and some even sleep eat, which must make calorie counting difficult.

Other sleep disorders can be even more bothersome. For instance, the Kleine-Levin Syndrome Foundation works with people who suffer from cyclical bouts of needing excessive amounts of sleep. Sufferers can sleep away an entire day and night and still wake up tired.

Other sleep disorders primarily affect children. For instance, the National Kidney Foundation's When Bed-Wetting Becomes a Problem states that five to seven million children in the United States alone struggle with bed-wetting. The good news is that most children outgrow this.

As life grows continually more complicated - cell phones ringing around the clock, beepers buzzing, and other devices nagging us whenever possible - settling into peaceful sleep might grow more elusive. But, fear not. You can always point your browser to Snuz and order a CD with just the right sleep-inducing sounds for you, whether your favorite lullaby is the rhythmic rumble of a clothes dryer or a humming window fan. So, sip some warm milk, pop in your favorite collection of monotonous noise, and, hopefully, sleep will visit you. Good night.