Study Notes on Biological Rhythms and Sleep

Introduction to Behavioral Neuroscience: Chapter 15 - Biological Rhythms and Sleep

This section provides an overview of biological rhythms and sleep, focusing specifically on circadian rhythms, their regulation, associated disorders, and societal implications. The content is compiled from OpenStax educational materials adapted for academic use.

Chapter Overview

Chapter Contributors

  • Chapter by: Megan M. Mahoney, PhD and Eric M. Mintz, PhD

  • Slides prepared by: Elizabeth D. Kirby

Chapter Outline

  • 15.1 What are Circadian Rhythms?

  • 15.2 Where are Rhythms in the Brain?

  • 15.3 Regulation of Sleep

  • 15.4 Disorders of Sleep and Circadian Rhythms

  • 15.5 Circadian Rhythms and Society

15.1 What are Circadian Rhythms?

Chronobiology

  • Definition: The study of biological clocks and biological rhythms within an organism, including cycles of gene transcription, hormone surges, and patterns of fatigue and alertness.

Types of Biological Rhythms

  • Circadian Rhythms: Approximately 24-hour cycle (e.g., sleep-wake cycle).

  • Ultradian Rhythms: Less than 24 hours (e.g., ocean tides).

  • Infradian Rhythms: Greater than 24 hours (e.g., moon cycles).

  • Circannual Rhythms: Approximately 1 year cycle (e.g., hibernation).

Characteristics of Biological Rhythms

Definitions

  • Rhythm: A repeating event that occurs with a regular pattern.

  • Level: Refers to the biological level such as gene expression, activity, hormones, etc.

  • Period: Duration of one cycle of the rhythm.

  • Amplitude: The magnitude of the rhythm.

  • Phase: The timing of the rhythm in relation to external events (day/night).

Endogenous Nature of Biological Rhythms

  • Biological rhythms are generated internally and are endogenous; for example, if kept in constant dim light, humans or animals still display a ~24-hour activity-rest/sleep cycle, known as the free-running period.

  • Free-running Period of Humans: Between 23.5 to 24.7 hours per day.

Chronotypes

  • Definition: Individual tendencies toward being alert at different times of the day (e.g., night owls vs. early birds).

Photic Phase Response Curve

  • Research in laboratory conditions shows that rodent activity increases at night. When placed in constant darkness, rodents maintain their 24-hour activity rhythm.

15.2 Where are Rhythms in the Brain?

Suprachiasmatic Nucleus (SCN)

  • Located in the hypothalamus, the SCN is referred to as the master clock of the body.

Lesion Experiments

  1. An intact hamster exhibits a 24-hour rhythm of activity.

  2. Lesioning the SCN causes the hamster to show arrhythmic activity (lack of rhythm).

  3. Transplanting fetal SCN tissue restores the free-running rhythm, but the hamster still cannot entrain to light.

Input from the Retina

  1. Blue light activates intrinsically photosensitive retinal ganglion cells (ipRGCs), which lead to stimulation of the SCN.

Outputs to Other Brain Regions

  • The SCN sends output signals to various areas including the hypothalamus and thalamus, affecting:
      - Body temperature
      - Locomotor activity
      - Sleep-wake cycles
      - Hormone release

Clock Genes and Biological Rhythms

  • Function of CLOCK and BMAL1 Genes:
      1. CLOCK and BMAL1 proteins bind to the promoters of per and cry genes and induce their expression.
      2. PER and CRY proteins then accumulate in the cytoplasm.
      3. Eventually, PER and CRY enter the nucleus and inhibit the action of CLOCK and BMAL1, reducing or blocking the expression of per and cry genes.
      4. Once PER and CRY proteins degrade, CLOCK and BMAL1 can re-bind, restarting the cycle of gene expression.

Connection to Melatonin

  1. SCN activation inhibits the pineal gland's secretion of melatonin— a hormone associated with sleep regulation.

  2. Light activates ipRGCs, leading to activation of the SCN, which in turn leads to reduced melatonin secretion from the pineal gland.

Applications of Melatonin

  • Melatonin is used as a medical treatment for:
      - Insomnia
      - Jet lag
      - Circadian rhythm disorders related to shift work
      - Supporting sleep in the elderly.

Test-like Question

  • Where would damage result in a disturbed ability to maintain the circadian rhythm of melatonin?
      - a) Visual cortex
      - b) Optic nerve
      - c) Suprachiasmatic nucleus
      - d) A and C
      - e) B and C

15.3 Regulation of Sleep

What Drives Sleep Patterns?

  • Process C: Circadian rhythm

  • Process S: Homeostatic pressure (the pressure to sleep increases with time spent awake).

Importance of Sleep

  • Reasons for Sleep:   - Restoration of physical and mental energy   - Protection from predators   - Support of learning and memory consolidation (plasticity)   - Reduction of energy demand and support for growth and development.

Measurement of Sleep

  • Polysomnography: This includes measuring brain waves (EEG), eye movements (EOG), and muscle movements (EMG).

Sleep Stages Revealed by Polysomnography

  • Three main stages: Wake, NREM (Non-Rapid Eye Movement), and REM (Rapid Eye Movement).

  • Hypnogram reveals that NREM is more likely to occur in the earlier parts of the sleep period while REM occurs more often in the latter half.

Brain Circuits Regulating Sleep

  • Sleep transitions rely on brain regions that promote wakefulness or sleep, which inhibit one another, functioning as a flip-flop switch.

  • Wake-Promoting Regions Include:
      - TMN (Tuberomammillary nucleus, histamine)
      - Locus coeruleus (norepinephrine)
      - Raphe nuclei (serotonin)

  • Sleep-Promoting Regions Include:
      - VLPO (Ventrolateral preoptic nucleus, GABA, galanin)
      - LDT/PPT (Laterodorsal tegmental nucleus, acetylcholine).

15.4 Disorders of Sleep and Circadian Rhythms

Common Sleep Problems

  • Inadequate sleep duration

  • Fragmentation of sleep periods

  • Sleepiness at inappropriate times

  • Difficulty falling asleep when desired.

Non-24 Hour Sleep/Wake Disorder

  • Occurs when an individual’s internal clock does not synchronize with external cues; retina damage can be a potential cause.

Narcolepsy

  • Features include excessive daytime sleepiness, cataplexy (loss of muscle tone), visual/auditory hallucinations during sleep onset or waking, and sleep paralysis.

  • Night sleep is marked by frequent awakenings and disordered stages.

  • Daytime sleep includes rapid onset of REM sleep.

15.5 Circadian Rhythms and Society

School Start Times

  • A significant number of U.S. middle and high schools begin the school day too early, with 5 out of 6 starting before 8:30 AM.

  • Recommendations from the American Academy of Pediatrics suggest that schools should start no earlier than 8:30 AM to facilitate adequate sleep, particularly since teens require at least 8 hours and younger students need at least 9 hours of sleep.

  • Two-thirds of U.S. high school students sleep less than the recommended amount on school nights.

Impact of Insufficient Sleep

  • Lack of sufficient sleep can lead to:
      - Increased incidence of being overweight
      - Reduced physical activity
      - Symptoms of depression
      - Engagement in risky behaviors (e.g., substance abuse)
      - Poor academic performance.

Social Jetlag

  • Defined as the mismatch between sleep schedules during the week versus weekends, resulting in poor sleep quality and chronic partial sleep deprivation.

  • It's linked to negative outcomes such as reduced attention, increased fatigue, poor performance, and associations with obesity, diabetes, and metabolic dysfunction.

Implications of Sleep Regulation in Medicine

  • On July 2003, the US Accreditation Council for Graduate Medical Studies limited hours for interns and residents to ensure better sleep:
      - No more than 80 hours per week was permitted, with 24-hour maximum shifts and at least one day off every seven days.
      - A study showed a 0.25% decrease in overall mortality and a 3.7% lower relative risk of death following these regulations.