Sleep contrasts with wakefulness, in which state there is an enhanced potential for sensitivity and efficient responsiveness to external stimuli.
Sleep is a normal, reversible, recurrent state of reduced responsiveness to external stimulation that is accompanied by complex and predictable changes in physiology. These changes include coordinated, spontaneous, and internally generated brain activity as well as fluctuations in hormone levels and relaxation of musculature. A succinctly defined specific purpose of sleep remains unclear, but that is partly because sleep is a dynamic state that influences all physiology, rather than an individual organ or other isolated physical system. Sleep contrasts with wakefulness, in which state there is an enhanced potential for sensitivity and efficient responsiveness to external stimuli. The sleep-wakefulness alternation is the most-striking manifestation in higher vertebrates of the more-general phenomenon of periodicity in the activity or responsivity of living tissue.
There is no single perfectly reliable criterion for defining sleep. It is typically described by the convergence of observations satisfying several different behavioural, motor, sensory, and physiological criteria. Occasionally, one or more of those criteria may be absent during sleep (e.g., in sleepwalking) or present during wakefulness (e.g., when sitting calmly), but even in such cases there usually is little difficulty in achieving agreement among observers in the discrimination between the two behavioral states.
THE SCIENCE BEHIND SLEEP
An internal “body clock” regulates your sleep cycle, controlling when you feel tired and ready for bed or refreshed and alert. This clock operates on a 24-hour cycle known as the circadian rhythm. After waking up from sleep, you’ll become increasingly tired throughout the day. These feelings will peak in the evening leading up to bedtime.
This sleep drive – also known as sleep-wake homeostasis – may be linked to adenosine, an organic compound produced in the brain. Adenosine levels increase throughout the day as you become more tired, and then the body breaks down this compound during sleep.
Light also influences the circadian rhythm. The brain contains a special region of nerve cells known as the hypothalamus, and a cluster of cells in the hypothalamus called the suprachiasmatic nucleus, which processes signals when the eyes are exposed to natural or artificial light. These signals help the brain determine whether it is day or night.
As natural light disappears in the evening, the body will release melatonin, a hormone that induces drowsiness. When the sun rises in the morning, the body will release the hormone known as cortisol which promotes energy and alertness.
THE IMPORTANCE OF GETTING ENOUGH SLEEP
For most adults, at least seven hours of sleep each night is needed for proper cognitive and behavioural functions. An insufficient amount of sleep can lead to serious repercussions. Some studies have shown sleep deprivation leaves people vulnerable to attention lapses, reduced cognition, delayed reactions, and mood shifts.
It’s also been suggested that people can develop a sort of tolerance to chronic sleep deprivation. Even though their brains and bodies struggle due to lack of sleep, they may not be aware of their own deficiencies because less sleep feels normal to them. Additionally, lack of sleep has been linked to a higher risk for certain diseases and medical conditions. These include obesity, type 2 diabetes, high blood pressure, heart disease, stroke, poor mental health, and early death.
Adults who do not receive a sufficient amount of sleep each night can implement some positive lifestyle and sleep habits in order to log the needed seven to nine hours. These include the following:
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- Establish a realistic bedtime and stick to it every night, even on the weekends.
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- Maintain comfortable temperature settings and low light levels in your bedroom.
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- Keep a comfortable sleep environment by ensuring you have the best mattress, best pillows, and best sheets for your sleep preferences and body type.
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- Consider a “screen ban” on televisions, computers and tablets, cell phones, and other electronic devices in your bedroom.
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- Abstain from caffeine, alcohol, and large meals in the hours leading up to bedtime.
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- Refrain from using tobacco at any time of day or night.
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- Exercise during the day; this can help you wind down in the evening and prepare for sleep.
HOW DOES THE BODY REGULATE SLEEP?
The body regulates sleep with two key drivers: sleep-wake homeostasis and the circadian alerting system.
Sleep-wake homeostasis. This technical term describes something most of us know implicitly from experience: the longer you’re awake, the more you feel a need to sleep. This is because of the homeostatic sleep drive, the body’s self-regulating system in which pressure to sleep builds up based on how long you’ve been awake. This same drive causes you to sleep longer or more deeply after a period of insufficient sleep.
The circadian alerting system. Part of your body’s biological clock, circadian rhythms last roughly 24 hours and play a central role in numerous biological processes, including sleep. Light exposure is the biggest influence on circadian rhythms, encouraging wakefulness during the day and sleepiness at night.
These two factors directly affect how much your body feels a need for sleep, reflecting your biological clock, the time of day, your light exposure, and how long you’ve been awake.
In addition, a wide range of external factors can influence sleep-wake homeostasis and the circadian alerting system. For example, stress or hunger may disrupt your normal process of sleep regulation. Caffeine intake or exposure to light from electronic devices are other examples of how behavioral choices can alter the body’s underlying systems for managing sleep.
These multifaceted processes are managed by several parts of the brain including the hypothalamus, the thalamus, the pineal gland, the basal forebrain, the midbrain, the brain stem, the amygdala, and the cerebral cortex. The fact that so many parts of the brain are involved in wakefulness and sleep, including the sleep stages, is a further demonstration of the biological complexity of sleep.
What Chemicals and Hormones Regulate Sleep?
Numerous chemicals and hormones are involved in the mechanics of sleep-wake homeostasis and the circadian alerting system. Shifting between wakefulness and sleep creates changes in thousands of neurons in the brain and a complex signalling system that generates specific reactions in the body.
To date, there is much that is still unknown about the intricate processes that control sleep, but researchers have discovered some substances that appear to be important cogs in the machinery of sleep.
A chemical called adenosine is believed to play a central role in sleep-wake homeostasis. Adenosine builds up when we’re awake and appears to increase sleep pressure. Caffeine, on the other hand, suppresses adenosine, which may explain part of how it promotes wakefulness.
Neurotransmitters are chemicals that send signals within the nervous system to activate or deactivate certain cells. Examples of neurotransmitters involved in promoting wakefulness or sleep include GABA, acetylcholine, orexin, and serotonin.
Hormones also play an integral role in signalling and regulating sleep-wake states. Melatonin, which promotes sleep and is naturally produced as light exposure decreases, is one of the best-known hormones related to sleep. Other important sleep-related hormones include adrenaline, cortisol, and norepinephrine. Sleep can also affect the production of vital hormones, such as growth hormone as well as leptin and ghrelin that regulate appetite, which may exert influence on sleep-wake homeostasis and circadian rhythms.
The function of these chemicals and hormones may be different in some individuals based on their genetics, which is why certain sleep disorders like sleep apnea may run in families. Environment and lifestyle choices may also influence the chemical and hormonal signaling responsible for sleep.
Sources
https://www.britannica.com/science/sleep
https://www.sleepfoundation.org/how-sleep-works/why-do-we-need-sleep
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