Losing sleep can hobble reflexes and fog the mind. Study volunteers deprived of just a couple of hours for a few nights in a row have slowed reaction times and faltering concentration. The consequences can be deadly. Lack of sleep causes innumerable road accidents each year.

A sound sleep seems to be key to a healthy heart. "As soon as people drift off, levels of hormones that stimulate the sympathetic nervous system during the waking hours, called catecholamines, begin to ebb," says research psychiatrist Michael Irwin. The result: Blood pressure falls. Heart rate slows. Blood vessels relax, making it easier for blood to flow. "Our hearts, in other words, get a much deserved rest," says Irwin.

Not so when study subjects are deprived of sleep. Catecholamine levels can actually climb, making the heart work harder than normal -- risky.

Sleeplessness can knock the stuffing out of the body's immune defenses as well. Immune cells may be responsible for commanding sleep, in fact. And when we don't obey their orders, our germ-fighting cells begin to stumble.

One or two nights of bad sleep probably don't pose much danger. "However, chronic sleep deprivation might," says neuroscientist Carol Everson. She found that when rats are sleep deprived over a three-week period, bacteria that normally reside in the gut begin to spread to organs and the bloodstream, making the animals ill. Death can result. "That's worrying because their basic biological needs are lot like ours," says Everson.

How much sleep do you need to steer clear of trouble?
The experts' advice is to aim for seven to nine hours a night. If you find yourself tossing and turning, cut out coffee, tea, cola and alcohol too. If you still have trouble sleeping, talk to your doctor.

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