As suggested by its name, PLMD is an involuntary movement disorder. (An older name, nocturnal myoclonus , is rarely used anymore.) People with this problem experience sudden, involuntary leg movements during the night, such as kicking or jerking. The difference between this and restless leg syndrome is that, unless the kicking wakes you up, you don't know you're doing it. You don't experience the tingling and discomfort that leads you to consciously move your legs, as with restless leg syndrome. At least 80 percent of people with restless leg syndrome have PLMD, but the reverse isn't true.

This clock is a small part of the brain called the suprachiasmatic nucleus of the hypothalamus. It sits just above the nerves leaving the back of our eyes. Light and exercise "reset" the clock and can move it forward or backward. Abnormalities related to this clock are called circadian rhythm disorders ("circa" means "about," and "dies" means "day").

What to do: Talk to experts about sleeplessness, and you'll be told to practice good "sleep hygiene." What this means is that you need to take your lack of sleep seriously and look at your sleep habits and physical surroundings to see what might be preventing you from sleeping well. Start with your evening habits: What do you do in the hours before bed? Eliminate late-night eating, drinking, and computer use and your chances of falling asleep quickly and sleeping soundly are much greater. Use the last hour before bed to do things that relax you, like taking a warm bath, meditating, or reading.

The natural internal clock that controls our 24-hour cycle of sleep and waking, circadian rhythms are easily upset by changes in schedule, and they're greatly affected by light and darkness. Jet lag is the best known circadian rhythm disorder, but this sensitive inner clock can also be disrupted by changes in routine resulting in an erratic sleep schedule.