Does the body adapt well when it's pressed to do more on less rest? Is daytime drowsiness a harmless problem to be remedied with an extra cup of coffee -- or do we risk more than an embarrassing nod-off at the conference table?

Bedtime building
Sleep engages an untold number of biochemical and physiological changes that have a profound impact on our physical and psychological health.

The endocrine system, for example, gets busy secreting hormones that regulate energy and control metabolism. Growth hormones that affect muscle mass in adults (and growth rates in children) are released, and levels of cortisol increase to help manage stress and depression, promote wakefulness, and regulate blood pressure. The immune system appears to be fortified with sleep as well, helping the body ward off disease and recover from injury. At the cellular level, sleep is associated with a boost in protein production, which is vital for cell growth and for the repair of cells damaged by stress and ultraviolet rays.

Some researchers have reasoned that sleep is related to survival, since it keeps a person or any other sleeping animal still and out of danger for a significant period of time every day. True or not, sleep-induced immobility provides the brain with an opportunity to do its own custodial work -- storing memories, cataloging experiences, establishing connections -- while the rest of the body reduces its neurological demands.

Awake ... but not all the way
Given such vital goings-on under the veil of night, it's not hard to understand how a lack of sleep impacts the waking hours. We know instinctively upon waking whether we've slept enough or face a day of feeling drowsy and irritable. These are not just annoyances, either, but tip-of-iceberg indications of sleep deprivation.

Sleepiness has a direct effect on cognitive functioning, including the ability to concentrate and store memories. It makes us more susceptible to emotional triggers and less able to call on the executive brain functions that control judgment, attention and decision-making. Sleepiness also decreases motor skills and reaction time. An increasing number of studies are also drawing associations between insufficient sleep and diseases, including diabetes, hypertension, obesity and depression.

More than a snore
If you regularly have trouble sleeping, you may suffer from one of more than 80 disorders associated with sleep and wakefulness. Insomnia, restless legs, sleepwalking and nighttime teeth-grinding are just a handful of the conditions that can affect our nights and days.

In health circles, attention has turned to the treatment of sleep apnea, a potentially dangerous condition in which breathing stops during sleep. If there's snoring in your bedroom, consult a physician to learn whether you or your bedmate are among the 18 million Americans with the condition. Snoring may be bothersome, but sleep apnea, at its worst, is associated with cardiovascular disease. The disruption caused by sleep apnea can also represent significant risk in the workplace. Earlier this year, the Federal Aviation Administration mandated testing for sleep apnea among airline pilots.

Sleep math
So, how much sleep do we need to maintain good physical and mental health?

It takes long, uninterrupted blocks of rest to initiate and complete the sleep processes that support good health. After about an hour and a half at rest, a sleeping body enters its first sleep cycle of the night. The cycle takes the brain and body through four stages of sleep, and each stage is understood to support a unique set of health functions. The entire cycle takes about 90 minutes, and adults need about four cycles every night.

Totaling the time it takes to enter productive sleep, complete the four cycles, and come back around to full wakefulness after the fourth cycle, we're right around eight hours. That's why the leading authorities on sleep, from the National Sleep Foundation to the Cleveland Clinic to the Centers for Disease Control and Prevention, recommend seven to nine hours for adults each night.

It's a myth, however, that we need less sleep as we age. While there's a significant drop between infancy and adolescence, the required number of hours for an adult plateaus after the age of 18 or so. Whether you're 19 or 90, your system is probably calling out for rest if you're getting fewer than seven hours per night. The flood of commitments, interests and pleas for our attention may keep us upright and awake for ever-longer days. But that doesn't mean we need sleep any less.