The Importance of Sleep and Defining Sleep Cycle

Sleep is a natural biological process that allows the human body and mind to rest and recover. It involves the repair of body tissues, restoration of energy, balancing of hormones, as well as the processing of emotions by the brain. Good quality of sleep is equally as important as food, water or exercise for complete wellness of the body. This essential process is divided into stages, called sleep cycles, that determine the overall quality of sleep. A sleep cycle is the repetitive pattern of stages that the body and mind experience, the changes that occur in the body, involving certain physical functions. The sleep cycle is therefore important for developing mental and physical health, along with ensuring that both are maintained properly.

Duration and Stages of Sleep

The typical cycle can last about 90 to 120 minutes, with repetitions of 4-6 cycles per night, depending on each individual’s total duration and its quality. The cycle irrefutably includes different stages of Non Rapid Eye Movement (NREM) and Rapid Eye Movement (REM).

Stages of sleep

• NREM Stage 1
  Here, the body begins to relax so the sleep is faint. The brain is responsible for slowing down physiological activities, such as decrease in muscle tension, less eye movement, and slower breathing. Alpha and theta waves are intertwined with this stage, where the earlier phase produces alpha waves with varying frequencies of 8 to 13 Hertz, representing the relaxation phase. If a person continues to sleep through stage 1, there is a rise in theta waves, that have frequencies from 4-7 Hertz. Overall, during this time, it is easy to wake people up.
• NREM Stage 2
  Stage 2 involves deeper sleep, where the body cools down and brain activity is impeded. Theta waves continue to dominate the brain activities as well. Both the heart rate and breathing rate reach a stability, and sleep spindles are produced by the brain that aid in vitalizing memories. Additionally, they are also responsible for sorting out information that the brain has received throughout the day. K-complexes (biphasic waves) are also produced by the brain, that repress cortical activity to maintain sleep when encountered by a stimulus.
• NREM Stage 3
  This is the third and final stage of NREM, where the restoration of physical health occurs. Stage 3 is associated with delta waves that have very low frequency referring to deep sleep. In this stage, a person’s heart rate decreases slowly, damaged body tissues are repaired, and immune system is being boosted. It is said to be the stage where waking up someone is the most difficult. When they wake up however, they tend to feel refreshed.
• REM
  The fourth and final phase of the stages is the REM, which has a longer duration that includes heightened brain activity, dreaming and memory consolidation. Here, the eyes move quickly behind the eyelids, vivid dreaming occurs often and the brain activity is high. REM also plays a role in supporting emotional and mental health, and usually occurs more in the second half of the night, while assisting in brain functions.

The transition in sleep cycles occurs through a natural repeating pattern that is driven by the brain’s surveillance of the cycle. A sleep cycle begins with NREM moving from light to deep sleep, switching to REM, then resetting the whole cycle again. All these shifting and transitions are controlled by brainstem and hypothalamus, which monitor arousal, hormones and circadian rhythms and ensure the smooth progression through each stage.

Regulating the Sleep Cycle by Circadian Rhythm

What is the Circadian rhythm?

The circadian rhythm is the body’s natural oscillation that is repeated every 24 hours. This is the body’s internal 24-hour clock, that is in charge of regulating the sleep cycle. Alertness of the body is also linked with this, since it functions by responding to changes in light in an individual’s surroundings.

The superchiasmatic nucleus (SCN) in the hypothalamus acquires information about light through the retinohypothalamic pathway, where cells in the retina transmit signals to the SCN through the optic nerve (cranial nerve II).

During the day, activation of SCN leads to the use of inhibitory neurotransmitter GABA to the paraventricular nucleus (PVN), reducing activity in the sympathetic nervous system. This means that the pineal gland is not stimulated and melatonin is not secreted, so the body stays awake.

At night, the retinal signals decrease, so the SCN is no longer activated. However, the PVN is functional, and it sends signals to the superior cervical ganglion, stimulating the sympathetic nervous system. Pineal gland now releases melatonin, preparing the body for sleep.

[source: National Library of Medicine]

Alteration of Sleep Structure

Age and lifestyle

Ageing introduces shifts in the circadian rhythm, causing older adults to experience tiredness earlier than others in the evening. Changes in sleep schedules due to travelling or work load may cause them to have trouble falling asleep. They may also begin frequent napping, where longer naps can disrupt their sleep at night. Lifestyle has a significant impact on the quality of sleep as well. The food that an individual is consuming, along with their physical activity determines how good they are able to sleep throughout the night. Heavy meals or excessive screen time often make it difficult to fall asleep while working late or irregular hours disturb the body’s natural sleep rhythm.

Stress and health conditions

Stress plays an important role in influencing sleep quality, where hormones like cortisol are secreted, that reduce the amount of REM sleep. Mental health conditions such as depression, PTSD or anxiety often make sleeping difficult due to hyper arousal state or restless thoughts. Additionally, physical conditions like heart disease, diabetes or chronic pain also interrupt sleeping patterns, disrupting the cycle.

Consumption and surroundings

• Alcohol disturbs sleep quality, even when many find it easier to fall asleep after drinking. It delays REM sleep, reducing its duration, leading to overall poor quality of sleep.
• Caffeine blocks adenosine, a chemical responsible to make the body feel tired. Consuming caffeine late at night makes it harder to sleep, and it decreases deep sleep as well.
• Exposure of blue light reduces melatonin production, leading to delayed cycles. Poor sleep from blue light can also cause weight gain because it increases late-night snacking habits.

[source: Spline and joint, Thorne, Central Ohio]

Roles of REM, its Importance and Deep Sleep

REM and deep sleep are equally important and are linked with each other. REM sleep helps to improve emotional and mental health, while deep sleep helps in restoration of the body. REM improves learning and memory, enhances creativity, and braces mental health. On the other hand, deep sleep promotes bodily healing, balances hormones and builds up the immune system. A balanced combination of both is said to be a key towards refreshment.

Effects of Poor Sleep on Mood, Cognition and Performance

Poor sleep quality negatively impacts mood, cognition and performance by increasing irritation, reducing concentration and resulting in poor decision making. It also makes people prone to anger, since the brain’s emotion center, the amygdala in the temporal lobe becomes hyperactive, so sensitivity to stress heightens, causing minor inconveniences seem like big issues. Insufficient sleep also creates pathways for stress, since lack of proper rest compels the brain to lose concentration on daily tasks. This contributes to a decline in social life due to chronic tiredness, rising the risk of developing anxiety, or other conditions. Furthermore, poor sleep affects reaction time, causing poor judgement and difficulties with problem solving.

Sleep Disorders

Sleep disorders are conditions that disrupt the quality and quantity of sleep, and therefore cause impaired functioning in the daytime. Some common sleep disorders include:

1. Sleep apnea: Breathing often stops and restarts during sleep, causing loud snoring and tiredness during the day. This occurs when the upper airway is blocked or the brain doesn't send proper signals to the body to breathe.
2. Narcolepsy: It is a condition that causes sleepiness during the day. Symptoms include sleep paralysis, and sleep related hallucinations.
3. Insomnia: One experiences difficulty in falling or staying asleep. Nowadays, it is the most common sleeping problem amongst the population which may be due to increased stress.
4. Parasomnia: Parasomnias are a category of sleep disorders that involve abnormal movements, behaviors, emotions, and dreams that occur while sleeping. This may be related to other conditions, for instance, depression or sleep apnea.

Short and Long Term Effects of Disrupted Sleep

Continuation of disruption in the sleep cycle without any treatment may result in permanent damages to the body. Circadian rhythm disruption causes many pathologic conditions, including sleep disorders, cardiovascular problems, metabolic dysfunction, infertility, as well as cancer. Short term effects include decreased alertness, impaired judgment, memory problems, and mood changes. Physical health is also affected, where there are digestive issues, lethargy and a delay in wound healing.

Circadian dysfunction also reduces melatonin secretion which protects the cells against neurodegenerative disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), Pick’s disease, and progressive supranuclear palsy (PSP). Apart from that, there is also substantial evidence linking chronic circadian disruption in adolescence with increased drug-taking and a higher risk of affective disorders.

Sleep Hygiene and Habit for Maintaining Healthy Sleep

Sleep hygiene is a set of practices and routines that helps to get better sleep. These include:

• Maintain a consistent sleep schedule
• Steer clear of stressful, stimulating activities
• Consume meals at least 3-4 hours before sleeping
• Limit electronic usage close to bedtime
• Optimize the sleeping environment by keeping it quiet and dark
• Exercise regularly and have exposure to sunlight

Conclusion

Sleep is a process that shifts between NREM and REM, playing a vital role in balancing physical and mental health. The cycle is regulated by the circadian rhythm, while factors like age, health and lifestyle determine its quality as well. Therefore, it is important to maintain a healthy lifestyle and carry out sleep hygiene to ensure a better life without any physical or mental restraints.