Most people know that getting around 8 hours of sleep each night is important for optimal health. But what is our body actually doing while we rest to ensure our wellness? While many mechanisms of sleep are yet to be understood, research has found that our body performs these 10 essential functions while we snooze.
Sleeping consists of a cycle of rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. When we start to fall asleep, we enter NREM sleep, which makes up about 75% of the night. REM sleep happens after NREM sleep, approximately 90 minutes after falling asleep. This pattern repeats about every 90 minutes, with the REM sleep stage getting longer as the night goes on. The body carries out essential functions in both REM and NREM sleep. 
1. Cardiovascular System
In NREM sleep, your body’s heart rate and blood pressure gradually slow down as you sleep. In REM sleep, heart rate, blood pressure, and respiration may fluctuate, which is related to changes in sympathetic nervous system activity. These changes may promote cardiovascular health. [11, 13]
Insufficient sleep may stress the body, stimulating the release of more cortisol, norepinephrine, and other hormones during the day. This is helpful in maintaining blood pressure during the day, but these hormones may keep the body and cardiovascular system from resting at night. Studies have shown that sleep deprivation, with four hours of sleep each night, can decrease GABA (an inhibitory neurotransmitter) receptor expression in the hypothalamus. This can allow sympathetic excitation of glutamate and angiotensin II, which can impact blood pressure, heart rate, and sympathetic activity. These changes may ultimately impact cardiovascular function. [11, 13, 14]
Sleep may also benefit the body’s microvasculature. This includes the smallest capillaries, arterioles, and arteries, which operate in the exchange of nutrients and gasses. Disturbances in the autonomous nervous system, metabolism, and inflammation may harm the microvasculature. These metabolic and inflammatory changes occur before hypertension and insulin resistance, making poor microvascular function an early marker for both stress-related and metabolic-related cardiovascular disease. 
Studies have shown lower capillary density with both decreased quality and decreased duration of sleep. Sleep is important for ensuring microvascular health as well as overall cardiovascular wellness. 
2. Immune System Activity
While you’re sleeping, your body makes more cytokines, chemistry that helps the immune system fight infections and chronic inflammation. Insufficient sleep can reduce the body’s immune response to infections and vaccines. Getting a good night’s sleep can help the body use resources more effectively to fight infections. 
3. Appetite and Metabolic Regulation
Sleep is essential for the balance of the hunger and satiety hormones, leptin and ghrelin. Leptin helps to suppress appetite by telling you when you’re full, while ghrelin stimulates appetite. When we sleep well, leptin levels increase and the level of ghrelin decreases. Lack of sleep has been shown to increase ghrelin and thus cause us to feel hungrier and less satiated, as well as allowing for more time awake in which to consume foods. Additionally, people who sleep less are more likely to be overweight or obese, factors which are associated with diabetes and other chronic health conditions. 
Blood sugar and insulin levels also fluctuate throughout the night, potentially in accordance with the stages of sleep. Irregular or insufficient sleep may disrupt the blood sugar and insulin patterns, which may increase the risk for diabetes. 
4. Circadian Rhythms
Each person’s brain houses an internal clock, or circadian pacemaker, in the hypothalamus. Circadian rhythms influence many processes, including digestion, body temperature, and sleep. Natural human circadian rhythms follow an approximate 24-hour cycle, with important implications for our ability to sleep. 
Melatonin is commonly called the sleep hormone. Created from the inhibitory neurotransmitter serotonin, melatonin is secreted by the pineal gland according to the circadian rhythm and availability of serotonin. Melatonin production increases as it gets dark to help us feel sleepy naturally. During the day, melatonin secretion is low to aid in alertness; however, there is a smaller increase in the afternoon (between 1:00 pm and 4:00 pm) that may explain common daytime sleepiness. [9, 10, 13]
Like melatonin, cortisol follows the circadian rhythm. Cortisol is often considered to be the stress hormone, as it increases in response to stress. However, cortisol also plays important roles in the immune response, blood pressure, conversion of norepinephrine to epinephrine, and metabolism of carbohydrates, proteins, and fats. [6, 10] Therefore, changes in circadian rhythm and sleep time may impact cortisol activity.
Cortisol increases the production of glucose from protein (gluconeogenesis), helping to maintain optimal blood sugar levels. This is especially helpful when sleeping. During the night when the body is in a fasting state, cortisol increases glucose availability for the body to use for energy and repair. Thus, cortisol levels are highest in the morning after waking. As per the circadian rhythm, the morning peak gradually decreases throughout the day, reaching its lowest point around midnight. [6, 10]
7. Promotes Good Mood
Sleep is also important for ensuring optimal mood. Many people experience irritability, unhappiness, and stress with insufficient sleep.
8. Adenosine Breakdown
If you have put off sleep, or gone a few days with inadequate sleep, you know that the exhaustion will catch up with you. This is partly due to a buildup of adenosine. Adenosine is a neurotransmitter with many functions, including regulating sleep-wake homeostasis. Adenosine levels increase while awake, and the body breaks adenosine down during sleep. Without sleep, increasing adenosine levels can cause us to feel sleepy, eventually prompting us to give in to overwhelming sleepiness. [7, 13]
9. Other Hormones
During NREM sleep, the body secretes hormones such as human growth hormone (HGH). In children, HGH helps with growth and development. In people of all ages, HGH contributes to repair muscles, bones, and other tissue. Sex hormones are also released during sleep, including testosterone, luteinizing hormone, and follicle-stimulating hormone; these hormones aid in puberty and fertility. Sufficient sleep each night can help ensure optimal function of many body processes. 
10. Memory Maintenance
Sleep is also beneficial for communication between neurons and managing memories. It allows us to learn and acquire new memories, consolidate information, and incorporate new memories with the old memories. [1, 2, 5, 12, 13]
The neurons in our brains have a remarkable plasticity, or ability to change and strengthen. This challenges the homeostasis of cells and synapses, or the connections between neurons. Sleep may help to restore the homeostasis of synapses. During the day when we are learning and strengthening synapses, our cells have a greater demand for energy and delivery of resources. This plasticity can be a burden on neurons and other cells. Renormalizing synaptic strength during sleep helps to reduce this burden and restore neuronal selectivity and the ability to learn. This helps with consolidation and integration of memories. [2, 12] Cortisol, dopamine, and norepinephrine may also be beneficial for learning and memory consolidation during sleep. [1, 4, 5]
With reduced energy use as we sleep, our brain cells are allowed some downtime to do “housekeeping” functions. This may include resting of mitochondria, clearing extracellular space, recycling membranes, and replenishing of calcium and glutamate in certain areas. 
Sleep also aids in forgetting. While forgetting is often seen as a fluke or failure, some level of forgetting is necessary for optimal cognitive function. The brain is able to carry out selective “smart forgetting” to efficiently deal with the accumulation of unimportant details and to allow for adaptation. [2, 12]
Overall, getting regular, restful sleep is essential for wellness. Sleep promotes metabolic, cardiovascular, and immune system health. Sufficient sleep also helps to maintain optimal mood and memory. Inadequate sleep is associated with many chronic conditions as well as neurotransmitter and hormone imbalances. Consider speaking with your healthcare practitioner about your sleep habits, and start sleeping your way to better health!
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