DEAR DOCTOR K; I average about six hours of sleep each night. How much is enough, and how much do most people get? DEAR READER: Let's start with how much is enough.many large studies have found that people who average fewer than seven hours of sleep per night, or more than nine hours, have more health problems. However, these studies don't prove that too much sleep is the cause of their worse health. Instead, something else may be causing too little or too much sleep, and also causing health problems, For example, people who sleep more than nine hours per nihth tend to have higher rates of obesity, heart disease and depression. But it could be that their primary problem is depression,not excessive sleep.People with depression often spend more time sleeping. And they often eat too much and exercise too little, and thereby become obese.Obesity, poor diet and inadequate exercise all raise the risk of heart disease. Most experts are more convinced of the adverse effects of sleeping fewer than seven hours per night. How many people are, like you, averaging fewer than seven hours per night? A recent study publoshed by the US Centers for Disease Control and Prevention sheds light on that question. The study involved nearly half-a-million people from all 50 states and the District of Conlumbia. The good news was that most people (65%) reported that they averaged at least seven hours of sleep per night. The bad news was that 35% did not. That amounts to about 84