Understanding Sleep: Definitions, Measurements, and Development

Defining and Measuring Sleep; Sleep in Animals and Across Development

  • What is sleep?

    • Definition from dictionary:

    • Sleep is described as a reversible mental state characterized by low attention to the environment.

    • Typically accompanied by a relaxed posture and minimal movement.

Historical Conceptions of Sleep and Dreams

  • Historical perspectives:

    • Soul travels to another world: Predominant belief throughout much of human history.

    • General slowing down of the brain:

    • Concept developed around the machine era (circa 1800s).

    • Proposed that the brain clears contamination built-up while awake.

    • Various historical explanations for sleep:

    • Blood filling the brain while awake.

    • Vapors from food entering the brain while awake.

    • Build-up of "hypnotoxins" in the brain while awake.

    • Early 20th century - Behaviorism:

    • Viewed sleep as an idling state devoid of meaningful brain activity, leading to minimal sleep research during that era.

Measuring Events During Sleep

  • Key Developments:

    • Chance discovery of rapid eye movements (REM) by Eugene Aserinsky.

    • Introduction of Polysomnography, which involves several measurements:

    • Electroencephalogram (EEG): Measures electrical activity of the brain.

    • Electrooculogram (EOG): Measures eye movement.

    • Electromyogram (EMG): Measures muscle activity.

What is an EEG?

  • Definition and Function of EEG:

    1. Neurons firing cyclically create corresponding cyclical electrical potential (voltage) changes in surrounding tissue.

    2. These changes can be measured on the scalp.

  • EEG Mechanism:

    1. When many neurons in the same area fire synchronously, their voltage changes are amplified.

    2. In an EEG machine, an EEG “channel” consists of:

    • An electrical circuit between a scalp location above a brain area of interest and a neutral location (e.g., ear lobe).

    1. This circuit generates a record where:

    • The pen tracks synchronized neuron firing as voltage change amplitude and frequency (measured in Hertz, cycles per second).

Awakening and Sleep Polysomnogram Results

  • Awake Polysomnogram Results:

    • EEG Waves:

    • Waking state: Beta waves (16-25 Hz), irregular, and low amplitude.

    • Relaxed state: Alpha waves (8-12 Hz), regular, high amplitude.

    • Stage 1: Theta waves (3-7 Hz), irregular with low amplitude, subjectively drowsy with slow rolling eye movements and moderate EMG.

  • Stage Descriptions:

    • Stage 2 Sleep:

    • Characterized by theta waves and spindles, no eye movements, moderate EMG.

    • Stage 3 Sleep:

    • Delta waves (1-4 Hz), somewhat regular with high amplitude, no eye movements.

    • Stage 4 Sleep:

    • More intense stage 3 sleep with prominent delta waves; associated with phenomena like sleepwalking, night terrors, and teeth grinding.

Slow Wave Sleep (SWS)

  • Comprised of Stages 3 and 4 of NREM sleep, often referred to as Slow Wave Sleep (SWS) or delta sleep.

Properties of EEG Waves

  • Key Characteristics:

    • Regularity:

    • Regular waves reflect more synchronized neural firing with higher amplitude.

    • Irregularity:

    • Irregular waves reflect less synchronized neural firing with lower amplitude.

    • Fundamental Frequency:

    • The dominant wave frequency that carries the most energy and has the largest amplitude.

    • Sleep Spindle:

    • A brief (0.5-1.5 s), regular burst of 12-14 Hz waves, moderate amplitude; associated with the onset of Stage 2 sleep and may aid in memory consolidation.

Progression of Sleep Stages

  • The transition from NREM Stage 1 to Stage 4 is characterized by a progression from shallow to deep sleep:

    • Difficulty Awakening:

    • Stage 1 is easiest, whereas Slow Wave Sleep (SWS) is the hardest to wake from.

    • Neural Activity Levels:

    • SWS shows the lowest average neural activity and metabolic rate.

    • Proportion of People Continuing Tapping:

    • 100% awake.

    • 30% in stage 1.

    • 5% in stage 2.

    • 0% in SWS.

Characteristics of REM Sleep

  • EEG Characteristics:

    • Dominantly theta waves.

  • EOG Characteristics:

    • Rapid Eye Movements (REMs) occur exclusively during REM sleep.

  • EMG Characteristics:

    • Lowest electrical activity indicating full paralysis of major muscle groups (atonia).

  • REM Dreams:

    • These are typically the dreams that individuals remember upon waking.

Characteristics of Sleep in Animals

  • Sleep Patterns Across Species:

    • All mammals and many other vertebrates, such as birds, experience NREM sleep.

    • Only mammals and birds appear to exhibit REM sleep.

    • Reptiles show decreased brain activity during sleep that resembles NREM sleep, but no REM sleep.

    • Behavioral observations of lower movement and consistent posture in fish, worms, and insects suggest they may also experience sleep-like states.

  • Evolution of Sleep:

    • Evolutionary connections are drawn between birds and reptiles, highlighting the unique presence of REM sleep only in birds and mammals.

Sleep Time Across Mammalian Species

  • Variability in Total Sleep Time:

    Mammal

    Total Sleep Time/Nychthemeron (Hours)

    Giraffe

    1.9

    Roe Deer

    3.1

    Asiatic Elephant

    3.1

    Pilot Whale

    5.3

    Human

    8.0

    Baboon

    9.4

    Domestic Cat

    12.5

    Lab Rat

    13.0

    Lion

    13.5

    Eastern Chipmunk

    15.8

    Little Brown Bat

    19.9

  • Speculative Factors Influencing Sleep Time:

    • Caloric density of food.

    • Body size.

    • Prey versus predator status.

    • Habitat characteristics.

Torpor and Hibernation

  • Torpor:

    • Defined as regulated hypothermia, typically lasting less than 24 hours, seen in species such as birds and bats; low amplitude SWS is observed.

    • Function: Conserves energy during non-productive periods.

  • Hibernation:

    • A more extreme form of hypothermia lasting throughout winter, exhibited by animals such as bats and groundhogs.

    • Characterized by minimal recordable brain waves.

Sleep Adaptations in Marine Mammals

  • Challenges Faced:

    • Marine mammals and migratory birds face unique challenges on how to sleep while needing to surface for air or avoid crashing into the ocean.

  • Adaptation Mechanisms:

    • Possible solutions include:

    • No sleep at all.

    • Series of very short naps.

    • Actual adaptation involves unihemispheric sleep wherein one half of the brain sleeps while the other stays awake.

Specific Behaviors During One-Hemisphere Sleep

  • Dolphin Behaviors:

    • Dolphins swim side-by-side with slow, rhythmic breaths while one hemisphere sleeps.

  • Seal Behaviors:

    • Seals exhibit unihemispheric sleep when in water and may also sleep on land or while floating.