We don't need a scientist or a study to tell us that there's a price to be paid for losing sleep. You sag after lunch, or just plain feel crummy. Remember when your parents shooed you to bed with "Because you need your sleep, that's why!"? They were right--more than they knew.

Research now suggests that regular, ample sleep is one of those indispensables, ranking right up there with eating right and exercising. Recent experiments show that when you shortchange sleep, the human immune system produces fewer infection-fighting antibodies, making you vulnerable to disease. Researchers at the University of Chicago studied volunteers who slept four hours a night for six straight days. They found hormonal and metabolic systems in disarray. The conclusion: chronic sleep loss might hasten the onset and increase the severity of diabetes, high blood pressure and obesity. Another study linked lack of sleep to increased risk of heart attack.

There are also benefits to the brain. In Canada, researchers used a logic game called Wff N' Proof to test sleep's influence on "complex cognitive procedural" thinking. If people get drunk shortly before going to bed, after they learn the game in the afternoon, they do 40 percent worse the next time they play it than those who stayed sober. One explanation is that alcohol suppresses the REM (rapid eye movement) cycle of sleep, which you need in order to learn well. Similar experiments at Harvard have shown that people score better on memory tests if they sleep soundly for six hours the night after learning the task.

Memories are created by strengthening connections among networks of brain cells. Sleep may be the brain's way of tinkering with those connections--boosting some, dampening others. Each waking moment bombards your brain with sensations, thoughts and feelings. If your brain tried to store them all as memories, you might experience overload. Sleep may help edit out some of those impressions.

How much sleep do you need? A century ago most people got about nine hours a night. Researchers say that most of us need about eight hours. Nowadays we get about seven hours on average, and a third of us get by on six or less. The light bulb is partly to blame. Recent studies show that artificially illuminated evenings disrupt sleep and may produce sleep pathology in some people. Age is another factor. It's not true that you need less sleep as you get older, but it is true that you get less. The reasons: aches and pains, depression, medications, those nighttime trips to the bathroom because it is harder to fully empty the bladder.

How, in our plugged-in, overworked, present-day lives, can we get more sleep? There are some easy steps to take. Don't use alcohol as a sleeping pill; it disrupts normal sleep patterns. The bed should be a sleep- and sex-only zone. Avoid caffeinated coffee, tea, soft drinks and chocolate after noon. More fundamentally, we need to change our attitudes. Sleep isn't a form of laziness: it may make you smarter and healthier. We need to promote sleep as a necessary ingredient of a healthy life.