Читать книгу Understanding the Language of Silence - Sleep, Sleep Behavior and Sleep Disorders - Dr. Amrit Lal - Страница 8

Sleep is a biological need of all living beings. Sleep phenomenon has been continually preserved throughout evolution spanning millions of years and has survived many changes and challenges by way of sleep disorders that are potentially disruptive to sleep as a physiologic, psychological and social process.

Many myths abound about sleep. They refuse to die. Slumber scientists are still not certain about many facets of sleep behavior. What is known for sure is that sleep is a biological process, and that it is ubiquitous in the sense that its unique biology is common to all species of mammals, birds and reptiles. All animals apparently sleep even though their being unconscious and unresponsive to environment greatly raises the risk of their becoming another predator’s prey. Even cockroaches, bees and fruit flies appear to sleep. Sleep is thus a unique and ubiquitous phenomenon among mammals, birds and reptiles, although sleep patterns vary widely among different species of animal kingdom and within the same sect of species. For example, some mammals, such as humans sleep at night, while rats sleep during the day. Again, some species engage in uni-hemispheric sleep in the sense that one side of their brain is asleep while the other side is awake. Such sleep behavior has also been observed in birds that make long distance transoceanic flights and in aquatic animals like porpoises, dolphins and other marine mammals which allow them to sleep by alternatively switching off one hemisphere of their brain while the other remains in a waking stage and continue to swim and surface to breathe as well as to keep track of other members of their flock and watch for predators. Some sleep researchers wonder if sleep walking or somnambulism in humans is also a manifestation of this type of uni-hemispheric sleep.

Generally, human beings are able to get enough sleep if they are allowed to sleep at the natural times dictated by their body’s biological clock. Under normal circumstances, body’s internal clock that regulates circadian rhythm promotes a daily cycle of nighttime sleep for rest and recuperation and daytime alertness for activity and work. Also quite common is the physiologic mid-afternoon dip in alertness leading to daytime napping or siesta. “Sleep Architecture” or pattern of sleep changes with aging. This is a well-known fact about sleep behavior among humans.

An important aspect of sleep is that it progresses through multiple stages with dreamless period of light and deep sleep, and occasional periods of active dreaming (REM sleep). This cycle is repeated several times during the night. Generally, older people spend more time in lighter stages of sleep than in deep sleep even though their total sleep time, by and large, tends to remain constant

A little more sleep each night does help in many ways – health and productivity-wise. The need is physiologic. Sleep can increase serotonin (body’s own feel-good factor) level, which helps one think more clearly. Animal studies have demonstrated that sleep is essential for survival. For example, rats which normally live for two to three years, when deprived of sleep they can only survive for about 3 weeks. The longer an individual remains awake, stronger is the urge and need to sleep. In humans the pressure to sleep becomes overwhelming after 48 hours of being awake. This pressure to sleep defines the homeostatic component of sleep which can be defined as an ability of an organism to maintain an internal equilibrium. The precise mechanism underlying this pressure is not well understood except that the action of neurotransmitters and neurons or nerve cells located in the brainstem and at the base of the brain determine if we are asleep or awake. More recently, there is evidence that the molecule adenosine is an important sleep factor which appears to “kick start,” the lost sleep and in the process may induce sleep. These physiological differences in the states of sleep, dreaming versus quiet sleep were recognized as early as ancient Greece.

Sleep is actually a body’s default state - one is supposed to be asleep all the time. It occurs through the activation of the neurotransmitter gama-aminobutryric acid (GABA) in the brain. The reason we are not asleep all the time is that brain’s hypothalamus secrets a chemical called acetylcholine to keep us awake. For opposite effect, a chemical called adenosine builds up activity and hinders acetylcholine, which may trigger tiredness. As the day goes on, sleep-drive builds up and acetylcholine and other chemicals that promote wakefulness, decline. Meanwhile Melatonin (the key sleep-inducing hormone) level gradually rises about a couple of hours before bedtime, eventually overpowering what is left of wakefulness chemical - acetylcholine.

As detailed in Wikipedia:-

“Sleep is naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and more easily reversible than being in hibernation or coma.”

While still on the mode of defining sleep, a relevant entry on this in Encyclopedia Britannica reads:

“A succinctly defined, specific purpose of sleep remains unclear but this is partly due to the fact that sleep is a dynamic state that influences whole physiology, rather than an individual organ or other isolated physical system.”

The three salient characteristics of sleep - Reversibility, Recurrence and Spontaneity distinguish it from other states of human physiology marked by manifest inactivity. Sleep differs from state induced anesthesia or seen in people in coma because the later states are not readily reversible even by strong stimuli and they do not exhibit brain patterns characteristics of true sleep.

The mammals with the same correlates as human beings have the same sleep architecture marked by periodicity, reversibility and spontaneity. But down the evolutionary ladder these features are less likely to be observed.

The world great sleepers are sloths, opossums and armadillos; each clocking in 20 hours of sleep every 24 hours. In particular, perhaps two-toed sloth that inhibits tropical forests, hanging from branches upside down and feeding on leaves and shoots and fruits, is recognized as champion sleeper. No wonder it symbolizes indolence and laziness. Rats, hamsters and pigs snooze for about 12 hours a day. Shrews, elephants and giraffes, followed by horses, sheep, cows and guinea pigs sleep as little as two to four hours every 24 hours. Pet animals like cats and dogs sleep much longer that they would if they lived in the wild and hunt for food and escape from predators. Sleep is thus a universal phenomenon. Even plants sleep.