The risk of cancer may also be elevated in people who fail to get enough sleep. A Japanese study of nearly 24,000 women ages 40 to 79 found that those who slept less than six hours a night were more likely to develop breast cancer than women who slept longer. The increased risk may result from diminished secretion of the sleep hormone melatonin. Among participants in the Nurses Health Study, Eva S. Schernhammer of Harvard Medical School found a link between low melatonin levels and an increased risk of breast cancer.

　　A study of 1,240 people by researchers at Case Western Reserve University in Cleveland found an increased risk of potentially cancerous colorectal polyps in those who slept fewer than six hours nightly.

　　Children can also experience hormonal disruptions from inadequate sleep. Growth hormone is released during deep sleep; it not only stimulates growth in children, but also boosts muscle mass and repairs damaged cells and tissues in both children and adults.

　　Dr. Vatsal G. Thakkar, a psychiatrist affiliated with New York University, recently described evidence associating inadequate sleep with an erroneous diagnosis of attention deficit hyperactivity disorder in children. In one study, 28 percent of children with sleep problems had symptoms of the disorder, but not the disorder.

　　During sleep, the body produces cytokines, cellular hormones that help fight infections. Thus, short sleepers may be more susceptible to everyday infections like colds and flu. In a study of 153 healthy men and women, Sheldon Cohen and colleagues at Carnegie Mellon University found that those who slept less than seven hours a night were three times as likely to develop cold symptoms when exposed to a cold-causing virus than were people who slept eight or more hours.

　　Some of the most insidious effects of too little sleep involve mental processes like learning, memory, judgment and problem-solving. During sleep, new learning and memory pathways become encoded in the brain, and adequate sleep is necessary for those pathways to work optimally. People who are well rested are better able to learn a task and more likely to remember what they learned. The cognitive decline that so often accompanies aging may in part result from chronically poor sleep.

　　With insufficient sleep, thinking slows, it is harder to focus and pay attention, and people are more likely to make poor decisions and take undue risks. As you might guess, these effects can be disastrous when operating a motor vehicle or dangerous machine.

　　In driving tests, sleep-deprived people perform as if drunk, and no amount of caffeine or cold air can negate the ill effects.

　　At your next health checkup, tell your doctor how long and how well you sleep. Be honest: Sleep duration and quality can be as important to your health as your blood pressure and cholesterol level.