Tuesday, April 5, 2011
Why We Sleep
Scientists have yet to determine exactly why people sleep. However, they do know that humans must sleep and, in fact, people can survive longer without food than without sleep. And people are not alone in this need – all mammals, reptiles and birds sleep.
Scientists have proposed the following theories on why humans require sleep:
Sleep may be a way of recharging the brain. The brain has a chance to shut down and repair neurons and to exercise important neuronal connections that might otherwise deteriorate due to lack of activity.
Sleep gives the brain an opportunity to reorganize data to help find a solution to problem, process newly learned information and organize and archive memories.
Sleep lowers a person’s metabolic rate and energy consumption.
The cardiovascular system also gets a break during sleep. Researchers have found that people with normal or high blood pressure experience a 20 to 30% reduction in blood pressure and 10 to 20% reduction in heart rate.
During sleep, the body has a chance to replace chemicals and repair muscles, other tissues and aging or dead cells.
In children and young adults, growth hormones are released during deep sleep.
When a person falls asleep and wakes up is largely determined by his or her circadian rhythm, a day-night cycle of about 24 hours. Circadian rhythms greatly influence the timing, amount and quality of sleep.
For many small mammals such as rodents, sleep has other particular benefits, as it provides the only real opportunity for physical rest, and confines the animal to the thermal insulation of a nest. In these respects sleep conserves much energy in such mammals, particularly as sleep can also develop into a torpor, whereby metabolic rate drops significantly for a few hours during the sleep period. On the other hand, humans can usually rest and relax quite adequately during wakefulness, and there is only a modest further energy saving to be gained by sleeping. We do not enter torpor, and the fall in metabolic rate for a human adult sleeping rather lying resting but awake, is only about 5-10%.
More than 20% of Americans are shift workers who work and sleep against their bodies’ natural sleep-wake cycle. While a person’s circadian rhythm can not be ignored or reprogrammed, the cycle can be altered by the timing of things such as naps, exercise, bedtime, travel to a different time zone and exposure to light. The more stable and consistent the cycle is, the better the person sleeps. Disruption of circadian rhythms has even been found to cause mania in people with bipolar disorder.
"Sleepiness" cannot easily be quantified although such tests can be useful in getting some grip on it. Four common tests are used to measure and quantify effects of stimulants and symptoms of disorders.
Multiple sleep latency test (MSLT) – time to get to sleep
Maintenance of Wakefulness Test(MWT) – time to get to sleep
Wilkinson addition test – cognitive test
Digital symbol substitution – cognitive test
The two-phase model provides some guidance as to why people get sleepy – duration of prior waking and place in the circadian cycle.
Sloth as a sin
The "seven deadly sins" formulated by the mideaval monks included Sloth. The Bible in Proverbs 6:9 includes the line: "How long will you sleep, O sluggard? When will you arise out of your sleep?" But a more nuanced understanding of sloth sees it as a disinclination to labor or work. This isn't the same as the desire for healthy sleep. On the contrary, a person can't do work without rest periods and no one can operate at top performance without adequate sleep.
The Puritan work ethic can be adhered to and respect still paid to the sleep needs of healthy humans. It is wrong to see sleep as a shameful activity.
Sleep and the athlete - psychomotor vigilance
Psychomotor vigilance performance is important to athletic performance. Sleep experts use the word “vigilance” to roughly describe alertness and ability to do mental tasks. The vigilance is influenced by sleep’s homeostatic process. You can think of this process causing a build-up of pressure for sleep during wakefulness and a dissipation of pressure during sleep. The body’s circadian rhythm process produces a waxing and waning of pressure for wakefulness throughout the day.
This skill involves reaction time and sustained attention. It is needed for not only sports performance but also everyday activities such as driving. It is highly sensitive to sleep loss, often experienced by athletes on road trips, particularly after they cross multiple time zones.
Variations in sports performance may reflect normal ebb and flow of biological rhythms. Marked differences between time of training and time of competition also may dent an athlete's performance. Studies have shown that when athletes are allowed to sleep until "slept out," their mood, energy level, and sense of well-being increase. Does the well-rested athlete have a "secret" advantage? The value of sleep is hardly a secret.
Elite athletes often suffer problems due to domestic or occupational schedules that do not permit normal sleep schedules and to rapid travel across multiple time zones (jet lag). Endurance athletes often have a problem with immuno-suppression and chronic reduction in sleep can contribute to this. Indeed, even in non-athletes, sleep deprivation can suppress the immune system.
A related and largely unresolved question is the effect of exercise on sleep quality in regular people (non-athletes).
Conincidentally, sleep expert David Dinges has developed a "psychomotor vigilance test" to help organizations keep tabs on people in highly critical jobs.
Sleep Habits: More Important Than You Think
Chronic Sleep Deprivation May Harm Health
By Michael J. Breus, PhD
WebMD FeatureReviewed by Stuart J. Meyers, MD
Not sleeping enough and not sleeping well is not OK. As a matter of fact, there is quite a price to pay. It may surprise you to learn that chronic sleep deprivation, for whatever reason, significantly affects your health, performance, safety, and pocketbook.
There are many causes of sleep deprivation. The stresses of daily life may intrude upon our ability to sleep well, or perhaps we trade sleep for more work or play. We may have medical or mental-health conditions that disrupt our sleep, and be well aware that we are sleep-deprived.
However, it is critically important to realize that sleep deprivation is very often due to unrecognized sleep disorders. After a typical night's sleep, you may not feel restored and refreshed and be sleepy during the day, but be totally unaware that you are sleep-deprived or have a sleep disorder. You might think, "It's just the stress of work or the kids," or you might have "always felt this way" and had no idea that you should feel differently. This lack of awareness compounds the consequences, because so many people remain undiagnosed for years.
That said, let's look at the consequences of sleep deprivation.
In the short term:
Decreased Performance and Alertness: Sleep deprivation induces significant reductions in performance and alertness. Reducing your nighttime sleep by as little as one and a half hours for just one night could result in a reduction of daytime alertness by as much as 32%.
Memory and Cognitive Impairment: Decreased alertness and excessive daytime sleepiness impair your memory and your cognitive ability -- your ability to think and process information.
Stress Relationships: Disruption of a bed partner's sleep due to a sleep disorder may cause significant problems for the relationship (for example, separate bedrooms, conflicts, moodiness, etc.).
Poor Quality of Life: You might, for example, be unable to participate in certain activities that require sustained attention, like going to the movies, seeing your child in a school play, or watching a favorite TV show.
Occupational Injury: Excessive sleepiness also contributes to a greater than twofold higher risk of sustaining an occupational injury.
Automobile Injury: The National Highway Traffic Safety Administration (NHTSA) estimates conservatively that each year drowsy driving is responsible for at least 100,000 automobile crashes, 71,000 injuries, and 1,550 fatalities.
The good news for many of the disorders that cause sleep deprivation is that after risk assessment, education, and treatment, memory and cognitive deficits improve and the number of injuries decreases.
In the long term, the clinical consequences of untreated sleep disorders are large indeed. They are associated with numerous, serious medical illnesses, including:
High blood pressure
Psychiatric problems, including depression and other mood disorders
Attention Deficit Disorder (ADD)
Fetal and childhood growth retardation
Injury from accidents
Disruption of bed partner's sleep quality
Poor quality of life
National Sleep Foundation
Versus the recommended eight hours of sleep per night, American adults are sleeping an average of seven hours per night
One in three adults get 6.5 or less hours of sleep nightly
62% of American adults experience a sleep problem a few nights per week
43% of adults say they are so sleepy during the day that it interferes with their daily activities a few days a month
Overall, employees estimate that the quality and quantity of their work is diminished by about 30% when they are sleepy
Nearly one quarter (22%) of young adults are occasionally or frequently late for work due to sleepiness versus 11% of 30 to 64 year olds
13% of young adults admit to occasionally/frequently falling asleep at work
55% of those between the ages of 18 and 29 report that they "wake unrefreshed"
The percentage of young adults suffering from significant daytime sleepiness (33%) rivals that of shift workers (29%)
63% of tired drivers turn to caffeine and only 22% of drivers pull off the road to rest when drowsy
51% of all US adults reported they have driven while drowsy during the past year
24% of 18 to 29 year olds admitted to dozing off at the wheel at some point within the last year
Drowsy driving causes approximately 100,000 car crashes annually
68% of adults say that sleepiness interferes with their concentration and makes handling stress on the job more difficult
Sleep article's Webpage
Sleep and athlete Article's link
Sleep Stats Webpage