Dr. Michael Huckabee, professor at the University of Nebraska Medical Center, offers 3 not-so-well-known possible causes:

Night cramps

A sudden, intense pain in the calf from a muscle spasm can awake you from a dead sleep. Night cramps may occur once a year or less often, but some people have several a night.

Dehydration, electrolyte imbalance, nerve or muscle disorders and medication side effects can cause night cramps. So can working the muscles hard earlier in the day. If you can find out the cause, treatment might be as simple as drinking plenty of water or stretching your legs before you go to bed. If leg cramps disturb your sleep often, visit your doctor for help.

Restless legs

Do you get a powerful urge to move your legs? Do they feel uncomfortable when you sit or lie down? They may also have a creeping, crawling, tingling, or burning sensation. These can be symptoms of restless leg syndrome (RLS).

Sometimes, RLS is due to a health condition, such as anemia or to a side effect of some medications. In most cases, though, there is no known cause and it tends to run in families. Daytime exercise, relaxation techniques and other lifestyle changes can help. See your doctor for proper diagnosis and treatment.

Sleep paralysis

While fading off to sleep or waking up, you may experience a complete paralysis, unable to move or speak. You could also have a vivid nightmare or hallucination when your consciousness is drifting  between sleep and waking.

The exact cause of sleep paralysis is unknown but it may be related to an imbalance or overlapping of the stages of sleep. Stress, erratic sleep schedules and some medications may also be factors, and there is likely a genetic component. Despite how frightening it is, there are no serious health problems with sleep paralysis. A variety of treatments, from simple home exercises to prescription medications, have been shown to be effective.