Sleep Disorders: Disrupting The Natural Sleep-Wake Rhythm

The human sleep/wake cycle is controlled by two internal influences: sleep homeostasis and the circadian system. The circadian system is regulated by the suprachiasmatic nuclei, located in the hypothalamus, which is sensitive to light and dark signals from the eyes. When the circadian system is functioning normally, the body's sleep and wakefulness occur in harmony with the day-night cycle. However, various factors can interfere with this delicate balance, including age, genetics, neurodevelopmental differences, and external influences such as caffeine, artificial lighting, and air travel. Even a minor loss of sleep over a few days can lead to decreased performance, mood, and cognitive abilities. Understanding and maintaining healthy sleep/wake cycles are crucial for overall health and well-being.

Caffeine and artificial lighting

Caffeine is one of the most widely consumed stimulants in the world, with 90% of American adults consuming caffeine-infused beverages almost daily. Caffeine promotes wakefulness by blocking adenosine, a sleep-inducing chemical. Adenosine is a sleep-promoting chemical that is produced in the brain during our waking hours. Normally, adenosine builds up in the brain the longer you are awake. The more it builds up, the sleepier you become. When caffeine blocks this process, you remain alert and vigilant. However, this may also be why caffeine consumption can disrupt sleep. Caffeine can make you fall asleep later, sleep less overall, and make your sleep feel less satisfying.

Research by del Brutto et al noted secondary triggers to wakefulness, such as noise and artificial light, as well as interactions with other factors such as diet, which may interfere with sleep. The Atahualpa Project, using data collected in an Ecuadoran village with minimal external influences, found no impact of caffeine consumption on sleep patterns when controlling for other physiological variables. The researchers note that the absence of caffeine's impact on night sleep quality may be associated with the absence of other secondary triggers to wakefulness, such as noise or artificial light.

Artificial light has been shown to have dramatic effects on sleep, influencing circadian rhythm, melatonin production, and sleep cycles. Circadian rhythm is a 24-hour internal clock that coordinates a wide range of processes in the body, including sleep. This rhythm is controlled by a small part of the brain, known as the circadian pacemaker, that is powerfully influenced by light exposure. When light enters the eye, it is sensed by a special group of cells on the retina, which is carried to the brain and interpreted as information about the time of day. The brain then sends signals throughout the body.

The human sleep-wake cycle is the result of the interaction between two factors: the circadian drive for wakefulness and the homeostatic sleep pressure. The suprachiasmatic nucleus (SCN) of the hypothalamus acts as the principal circadian timekeeper, receiving illumination-dependent impulses from the eyes via the retinohypothalamic tract. The SCN triggers the release of cortisol and other hormones to help you wake up. But when darkness comes at night, the SCN sends messages to the pineal gland, which triggers the release of the sleep-inducing chemical melatonin.

In summary, caffeine and artificial light can interfere with the normal sleep-wake cycle by disrupting the balance between the circadian drive for wakefulness and the homeostatic sleep pressure. Caffeine blocks the sleep-inducing chemical adenosine, while artificial light disrupts the interpretation of light information by the circadian pacemaker in the brain.

Neurotransmitters and hormones

The sleep-wake cycle is controlled by a complex interaction between wakefulness-promoting and sleep-promoting nuclei in the hypothalamus and brainstem. Neurotransmitters and hormones play a crucial role in this process.

Neurotransmitters are chemicals that send messages to different nerve cells in the brain. Some neurotransmitters promote wakefulness, while others promote sleep. For example, serotonin (5-HT) functions predominantly to promote wakefulness and to inhibit REM sleep. However, under certain circumstances, it can also contribute to sleep propensity. Norepinephrine and histamine also promote wakefulness, while hypocretin (also called orexin) is involved in the control of sleep. A loss of hypocretin-producing neurons is linked to human narcolepsy.

Other neurotransmitters that promote sleep include gamma-aminobutyric acid (GABA) and galanin. GABA is an inhibitory neurotransmitter that inhibits the firing of cells involved in wakefulness. It is produced by certain groups of hypothalamic neurons and adjacent groups of basal forebrain neurons. Galanin is also produced by neurons in the hypothalamus and basal forebrain.

In addition to neurotransmitters, hormones also play a role in the sleep-wake cycle. The body's internal clock is controlled by the suprachiasmatic nucleus (SCN) in the hypothalamus, which is sensitive to signals of dark and light. In the morning, the SCN triggers the release of cortisol and other hormones to help you wake up. At night, the SCN sends messages to the pineal gland, which releases the hormone melatonin. Melatonin makes you feel sleepy and ready for bed.

The interaction between these neurotransmitters and hormones is complex and involves multiple brain regions and neuronal populations. This intricate balance between wakefulness-promoting and sleep-promoting factors ensures the proper regulation of the sleep-wake cycle.

Sleep disorders

There are over 70 recognised sleep disorders, which can be broadly categorised into four main types: insomnia, hypersomnia, parasomnias, and sleep-wake schedule disorders.

Age and genetics

Age plays a significant role in influencing sleep patterns. Newborns sleep sporadically, with sleep and feeding cycles occurring across the day and night. After three to four months, infants start to develop longer periods of consolidated sleep, along with daytime naps. As toddlers grow into young children, they typically transition to a single block of sleep at night, with fewer and shorter naps during the day. By the age of six or seven, most children have stopped napping entirely, and their sleep patterns resemble those of adults.

Throughout a person's lifespan, the amount of daily sleep gradually decreases. Newborns sleep for up to 20 hours a day, while adolescents require about nine hours of sleep for optimal functioning. Adults, including middle-aged individuals, generally need at least eight hours of sleep. However, older adults may struggle to obtain this recommended amount, even if their total sleep requirement remains the same or slightly decreases (around 6.5 to 7 hours per night). Aging brings about changes in sleep architecture, with older individuals experiencing more fragmented sleep, waking up more frequently, and spending less time in deep, dreamless sleep. They are also more likely to experience early morning awakenings and a shift in their sleep schedule, feeling tired earlier in the afternoon and waking up earlier.

Several factors contribute to the changes in sleep patterns with aging. One key factor is the decrease in melatonin production, a hormone that helps promote sleep and regulate circadian rhythms. Older adults also experience physical changes in their urinary systems, leading to an increase in nighttime urination (nocturia), which further disrupts their sleep. Additionally, mental and physical health conditions, such as depression, anxiety, heart disease, diabetes, and chronic pain, become more prevalent with age and can interfere with sleep.

Genetics also plays a crucial role in sleep patterns and disorders. Certain genetic variations or mutations in one or more genes can make individuals more vulnerable to the adverse effects of sleep loss. For example, people with a specific mutation may have a stronger drive to sleep and be less likely to develop insomnia, but they may struggle with jobs requiring extended shifts. Researchers are actively studying the impact of circadian genes on sleep, wakefulness, and the various cycles within these states.

Additionally, genetic factors can predispose individuals to specific sleep disorders. Delayed sleep phase syndrome (DSPS) is characterized by a late bedtime and wake time, often causing difficulties in fulfilling daily obligations. This condition may result from mutations in internal clock genes, which code for proteins that regulate timekeeping in our cells. Other sleep disorders with a potential genetic link include narcolepsy, sleep apnea, and hypersomnia. Narcolepsy, for instance, has been associated with a specific gene in mice and dogs, and researchers are working to identify the genes that increase the risk for sleep apnea in humans.

Travel and shift work

Travelling across time zones and working night shifts can significantly disrupt the sleep-wake cycle. This is due to a temporary desynchronization between the new time zone schedule and the individual's internal clock, known as circadian misalignment. The body's internal clock, or circadian alerting system, is highly sensitive to light and dark cues, which signal the body to wake up or go to sleep. When travelling across time zones, the body experiences a mismatch between its internal clock and the external environment, resulting in jet lag. Jet lag can cause excessive daytime sleepiness, nighttime insomnia, headaches, loss of appetite, gastrointestinal problems, and irritability or mild depression.

To minimize the effects of jet lag, travellers can manipulate their exposure to light. For eastward flights, it is recommended to avoid bright light during the late evening and night according to one's internal clock and seek light in the early morning. This helps to shift the internal biological clock towards alignment with the new time zone. However, adjusting to a new time zone can be challenging, and it may take time for the body to adjust to the local schedule.

Shift work, particularly night shifts, can also interfere with the sleep-wake cycle. About 20% of the full-time workforce in the United States participates in some form of shift work, and 10-40% of these individuals may experience Shift Work Sleep Disorder (SWSD). SWSD causes difficulties in falling asleep, staying asleep, and unwanted sleepiness. Working night shifts can lead to shortened and disrupted sleep during the day, as the body's signals to wake up and stay awake are strongest during daylight hours.

Additionally, social factors and family obligations can make it challenging for shift workers to maintain a consistent sleep/wake schedule. For example, a night shift worker may revert to a typical nighttime sleep/daytime awake schedule during their days off to spend time with family and friends. This constant shifting of the internal clock can result in the body's internal biological rhythms always remaining out of sync. To manage SWSD, individuals can make changes to their work routines and schedules, such as limiting the number of consecutive night shifts and taking extended time off between shifts.

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