💪Physiology of Motivated Behaviors Unit 11 – Sleep and Circadian Rhythms

Key Concepts and Terminology

• Sleep is a naturally recurring state of altered consciousness and reduced sensory activity characterized by relative inhibition of sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings
• Circadian rhythms are 24-hour cycles that are part of the body's internal clock running in the background to carry out essential functions and processes
• Homeostasis refers to the body's ability to maintain a stable internal environment despite changes in the external environment
• Sleep-wake cycle is an internal process that regulates the daily pattern of alternating wakefulness and sleep
• Suprachiasmatic nucleus (SCN) is a tiny region of the brain in the hypothalamus that contains about 20,000 neurons and coordinates many physiological processes
• Melatonin is a hormone that regulates sleep-wake cycles produced by the pineal gland increasing in the evening to promote sleep and decreasing in the morning to promote wakefulness
• Sleep pressure is the feeling of sleepiness and its intensity that gradually builds with every hour that you are awake

Biological Basis of Sleep

• Sleep is essential for survival and plays a crucial role in maintaining physical and mental health, cognitive function, and overall well-being
• Sleep is regulated by two main processes: the circadian process and the homeostatic process
  • The circadian process is controlled by the body's internal clock and regulates the timing of sleep and wakefulness throughout the 24-hour day
  • The homeostatic process is responsible for the accumulation of sleep pressure during wakefulness and its dissipation during sleep
• The hypothalamus, particularly the suprachiasmatic nucleus (SCN), plays a central role in regulating the sleep-wake cycle and circadian rhythms
• Neurotransmitters such as serotonin, norepinephrine, and acetylcholine are involved in promoting wakefulness while others like GABA and adenosine are involved in promoting sleep
• The pineal gland secretes melatonin, a hormone that helps regulate the sleep-wake cycle by increasing feelings of sleepiness
• Sleep is characterized by distinct stages that can be identified through electroencephalography (EEG), which measures electrical activity in the brain
• During sleep, the body undergoes various physiological changes, including changes in brain activity, heart rate, blood pressure, and muscle tone

Stages of Sleep

• Sleep is divided into two main types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep
• NREM sleep is further divided into three stages: N1, N2, and N3 (also known as slow-wave sleep)
  • N1 is the lightest stage of sleep, characterized by a transition from wakefulness to sleep, slow eye movements, and a decrease in muscle tone
  • N2 is a deeper stage of sleep, characterized by the appearance of sleep spindles and K-complexes on the EEG
  • N3 is the deepest stage of sleep, characterized by the presence of slow delta waves on the EEG and is important for physical restoration and memory consolidation
• REM sleep is characterized by rapid eye movements, vivid dreaming, and muscle atonia (temporary paralysis of voluntary muscles)
  • REM sleep is believed to be important for cognitive functions, such as memory consolidation and emotional regulation
• A typical sleep cycle lasts about 90-120 minutes and consists of a progression from NREM stages to REM sleep
• The proportion of time spent in each sleep stage changes throughout the night, with more slow-wave sleep occurring in the first half of the night and more REM sleep occurring in the second half

Circadian Rhythms and the Body Clock

• Circadian rhythms are endogenous, self-sustaining oscillations with a period of approximately 24 hours that regulate various physiological processes and behaviors
• The suprachiasmatic nucleus (SCN) in the hypothalamus acts as the body's master clock, synchronizing peripheral clocks found in various organs and tissues
• The SCN receives input from specialized photoreceptors in the retina, allowing it to synchronize with the external light-dark cycle
• Circadian rhythms influence numerous functions, including sleep-wake cycles, hormone secretion, body temperature, metabolism, and cognitive performance
• Disruption of circadian rhythms, such as through jet lag or shift work, can lead to sleep disturbances and other health problems
  • Jet lag occurs when the body's internal clock is out of sync with the external environment due to rapid travel across time zones
  • Shift work can lead to circadian misalignment, as the work schedule conflicts with the body's natural sleep-wake cycle
• Maintaining a consistent sleep schedule and exposure to light during the day can help entrain circadian rhythms and promote better sleep

Sleep Regulation and Homeostasis

• Sleep is regulated by the interaction between the circadian process and the homeostatic process
• The homeostatic process is responsible for the accumulation of sleep pressure during wakefulness and its dissipation during sleep
  • Sleep pressure increases with the duration of wakefulness and is thought to be mediated by the accumulation of adenosine, a neuromodulator that promotes sleep
  • During sleep, adenosine levels decrease, reducing sleep pressure and allowing for wakefulness
• The circadian process regulates the timing of sleep and wakefulness throughout the 24-hour day, promoting wakefulness during the day and sleep at night
• The interaction between the homeostatic and circadian processes determines the timing and duration of sleep
• Sleep deprivation leads to an increase in sleep pressure, resulting in a rebound effect characterized by increased sleep duration and intensity during subsequent sleep periods
• Chronic sleep deprivation can lead to a range of negative consequences, including cognitive impairment, mood disturbances, and metabolic dysregulation

Sleep Disorders and Their Impact

• Sleep disorders are conditions that affect the quality, timing, and duration of sleep, leading to daytime distress and impaired functioning
• Insomnia is a common sleep disorder characterized by difficulty falling asleep, staying asleep, or both, despite having adequate time and opportunity for sleep
  • Insomnia can be acute (short-term) or chronic (long-term) and may be caused by various factors, including stress, anxiety, depression, and poor sleep hygiene
• Sleep apnea is a disorder characterized by repeated pauses in breathing during sleep, leading to fragmented sleep and daytime sleepiness
  • Obstructive sleep apnea (OSA) is the most common type, caused by a collapse of the upper airway during sleep
• Restless leg syndrome (RLS) is a sensorimotor disorder characterized by an irresistible urge to move the legs, often accompanied by uncomfortable sensations, which can disrupt sleep
• Narcolepsy is a chronic neurological disorder characterized by excessive daytime sleepiness and sudden sleep attacks, often accompanied by cataplexy (sudden loss of muscle tone)
• Parasomnias are a group of sleep disorders characterized by abnormal behaviors or experiences during sleep, such as sleepwalking, sleep talking, and night terrors
• Sleep disorders can have significant impacts on physical and mental health, cognitive function, and overall quality of life, underscoring the importance of proper diagnosis and treatment

Factors Affecting Sleep Quality

• Sleep hygiene refers to a set of practices and habits that promote consistent, uninterrupted, and restorative sleep
  • Good sleep hygiene includes maintaining a regular sleep schedule, creating a comfortable sleep environment, avoiding stimulants (caffeine, nicotine) before bedtime, and engaging in relaxation techniques
• Light exposure plays a crucial role in regulating the sleep-wake cycle, with exposure to bright light during the day promoting wakefulness and darkness at night promoting sleep
  • Blue light emitted by electronic devices can suppress melatonin production and disrupt sleep, making it important to limit screen time before bed
• Stress and anxiety can lead to heightened arousal and difficulty falling asleep or staying asleep
  • Relaxation techniques, such as deep breathing, progressive muscle relaxation, and mindfulness meditation, can help reduce stress and promote better sleep
• Diet and exercise can also influence sleep quality
  • Regular physical activity can improve sleep quality and duration, but vigorous exercise close to bedtime may disrupt sleep
  • Heavy meals, alcohol, and caffeine consumption before bedtime can interfere with sleep quality
• Environmental factors, such as noise, temperature, and bedding, can also impact sleep quality
  • A quiet, cool, and dark bedroom environment is conducive to better sleep
  • Comfortable bedding and pillows can promote relaxation and improve sleep quality

Sleep's Role in Health and Behavior

• Sleep plays a vital role in maintaining physical health, with sufficient sleep duration and quality being associated with a reduced risk of chronic diseases, such as obesity, diabetes, and cardiovascular disease
  • Sleep deprivation can lead to metabolic dysregulation, increased inflammation, and impaired glucose tolerance, contributing to the development of these conditions
• Sleep is crucial for cognitive function, including attention, memory consolidation, and decision-making
  • REM sleep is particularly important for memory consolidation and emotional regulation
  • Sleep deprivation can lead to impaired cognitive performance, decreased alertness, and increased risk of accidents
• Sleep is essential for emotional well-being and mental health
  • Insufficient sleep is associated with an increased risk of mood disorders, such as depression and anxiety
  • REM sleep is thought to play a role in the processing and regulation of emotions
• Sleep is important for immune function, with adequate sleep being associated with a reduced risk of infections and improved immune response
  • Sleep deprivation can impair the immune system, making individuals more susceptible to illness
• Sleep also plays a role in regulating appetite and metabolism, with sleep deprivation being associated with increased hunger and cravings for high-calorie foods
  • Hormones involved in appetite regulation, such as leptin and ghrelin, are influenced by sleep duration and quality
• Addressing sleep disturbances and promoting healthy sleep habits can have far-reaching benefits for overall health, well-being, and quality of life