Sleep is defined as a state of unconsciousness from which a person can be aroused. In this state, the brain is relatively more responsive to internal stimuli than external stimuli. Sleep should be distinguished from coma. Coma is an unconscious state from which a person cannot be aroused. Sleep is essential for the normal, healthy functioning of the human body. It is a complicated physiological phenomenon that scientists do not fully understand.
Historically, sleep was thought to be a passive state. However, sleep is now known to be a dynamic process, and our brains are active during sleep. Sleep affects our physical and mental health, and is essential for the normal functioning of all the systems of our body, including the immune system. The effect of sleep on the immune system affects one’s ability to fight disease and endure sickness.
States of brain activity during sleep and wakefulness result from different activating and inhibiting forces that are generated within the brain. Neurotransmitters (chemicals involved in nerve signaling) control whether one is asleep or awake by acting on nerve cells (neurons) in different parts of the brain. Neurons located in the brainstem actively cause sleep by inhibiting other parts of the brain that keep a person awake.
Importance of Sleep
Animal studies have shown that sleep is necessary for survival. The normal life span of rats is 2-3 years. However, rats deprived of sleep live for only about 3 weeks. They also develop abnormally low body temperatures and sores on their tails and paws. The sores probably develop because of impairment of the rats’ immune systems.
In humans, it has been demonstrated that the metabolic activity of the brain decreases significantly after 24 hours of sustained wakefulness. Sleep deprivation results in a decrease in body temperature, a decrease in immune system function as measured by white blood cell count (the soldiers of the body), and a decrease in the release of growth hormone. Sleep deprivation can also cause increased heart rate variability.
For our nervous systems to work properly, sleep is needed. Sleep deprivation makes a person drowsy and unable to concentrate the next day. It also leads to impairment of memory and physical performance and reduced ability to carry out mathematical calculations. If sleep deprivation continues, hallucinations and mood swings may develop.
Release of growth hormone in children and young adults takes place during deep sleep. Most cells of the body show increased production and reduced breakdown of proteins during deep sleep. Sleep helps humans maintain optimal emotional and social functioning while we are awake by giving rest during sleep to the parts of the brain that control emotions and social interactions.
Rhythms That Influence Sleep
Biological variations that occur in the course of 24 hours are called circadian rhythms. Circadian rhythms are controlled by the body’s biological clock. Many bodily functions follow the biologic clock, but sleep and wakefulness comprise the most important circadian rhythm. Circadian sleep rhythm is one of the several body rhythms modulated by the hypothalamus (a part of the brain).
Light directly affects the circadian sleep rhythm. Light is called a zeitgeber, a German word meaning time-giver, because it sets the biological clock. A practical purpose has been proposed for the circadian rhythm, using the analogy of the brain being somewhat like a battery charging during sleep and discharging during wakefulness.
Body temperature cycles are also under control of the hypothalamus. An increase in body temperature is seen during the course of the day and a decrease is observed during the night. The temperature peaks and troughs are thought to mirror the sleep rhythm. People who are alert late in the evening (ie, evening types) have body temperature peaks late in the evening, while those who find themselves most alert early in the morning (ie, morning types) have body temperature peaks early in the evening.
Melatonin (a chemical produced by the pineal gland in the brain) has been implicated as a modulator of light entrainment. It is secreted maximally during the night. Prolactin, testosterone, and growth hormone also demonstrate circadian rhythms, with maximal secretion during the night.
Circadian rhythms can be affected to a certain degree by almost any kind of external stimulus, for example, the beeping of the alarm clock or the timing of meals. When we cross time zones, our circadian rhythms get disrupted leading to jet lag. It usually takes several days for our body rhythms to adjust to the new time.
Symptoms similar to those seen in people with jet lag are common in people who work during nights or work in shifts. Because these people’s wake time conflicts with powerful sleep-regulating cues like sunlight, they often become uncontrollably drowsy during work or may have difficulty falling asleep during their off time. Their biological clock wants to do one thing, while they are doing something entirely different. People working in shifts have an increased risk of heart, gastrointestinal, emotional, and mental problems. All these problems may be related to the disruption of the circadian sleep rhythm.
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