Table of Contents
Sleep is a special, regularly recurring state of the nervous system that fundamentally changes how information is processed and stored. It is not simply “turning off” the brain: during sleep, many neural networks are highly active, and specific patterns of activity support recovery, learning, and memory.
In this chapter, we focus on what is characteristic and biologically important about sleep, without re-explaining general nervous system structure or basic concepts of information processing and memory that are discussed elsewhere.
What Sleep Is (and Is Not)
Sleep can be defined biologically as:
- A reversible state of reduced responsiveness to external stimuli
- Characterized by typical brain activity patterns (sleep stages)
- Regulated by internal timing systems and chemical signals
Unlike coma or anesthesia, sleep:
- Is easily reversible (you can be awakened by a sound or light)
- Shows typical cycling between different stages
- Occurs naturally and rhythmically, often once every 24 hours (circadian rhythm)
During sleep, muscles are generally more relaxed, many reflexes are changed, and the threshold for awakening is higher. However, the brain continues to process some sensory information (for example, loud noises can wake you; a baby’s cry may awaken a parent while other sounds do not).
Why Animals Sleep
Almost all animals studied so far show some form of sleep or sleep-like states, even simple invertebrates.
From a biological perspective, sleep is costly:
- The animal is less responsive and more vulnerable to predators.
- Time is spent not searching for food, mates, or territory.
The fact that sleep is maintained across evolution, despite these costs, suggests major benefits:
- Restoration and recovery
- Cellular repair (especially in the brain)
- Clearance of metabolic waste products
- Energy saving
- Reduced metabolic rate and body temperature in many species
- Brain function optimization
- Stabilization of neural networks
- Reorganization of synaptic connections
- Support of learning and memory
- Development
- In young animals, sleep is important for brain maturation and growth
These functions are inferred from:
- What happens during sleep (brain activity patterns, hormone release, metabolic processes)
- What happens when sleep is lacking (cognitive, emotional, and health problems)
Sleep Stages
During a typical human night, sleep is not uniform. It cycles through several stages, distinguishable by characteristic patterns of brain electrical activity (measured by EEG), eye movements, and muscle tone.
Two main types:
- Non-REM (NREM) sleep
- REM (rapid eye movement) sleep
Non-REM Sleep
Non-REM sleep is often divided into several stages (commonly N1, N2, N3):
- N1 (light sleep)
- Transition from wakefulness to sleep
- Muscle tone decreases, eyes may slowly roll
- Brief awakenings are common; people often deny having been asleep
- N2 (stable light sleep)
- Characterized by specific EEG patterns (sleep spindles and K-complexes)
- Reduced responsiveness to surroundings
- Often the largest part of total sleep time in adults
- N3 (deep sleep, slow-wave sleep)
- Dominated by slow, high-amplitude brain waves
- Hardest to wake someone from this stage
- If awakened, people feel disoriented for a short time (“sleep inertia”)
- Linked to physical restoration and some types of memory consolidation
REM Sleep
REM sleep is physiologically very different from NREM:
- Rapid eye movements under the closed eyelids
- Low muscle tone: most skeletal muscles are strongly inhibited (motor “paralysis”)
- Active brain: EEG resembles wakefulness in many aspects
- Vivid dreaming is especially common in REM
The combination of an active brain and a largely paralyzed body is characteristic of REM sleep. This prevents the acting out of dreams in most cases.
Sleep Cycles
In humans, NREM and REM alternate in cycles:
- One full cycle (from light NREM → deeper NREM → back up → REM) lasts about 90 minutes on average.
- Early in the night:
- More deep N3 sleep
- Short REM periods
- Later in the night:
- Less N3 sleep
- Longer REM phases
This distribution suggests that different parts of the night serve different functions, for example stronger physical restoration early (N3) and more intense processing of certain memories and emotional experiences later (REM).
Biological Regulation of Sleep
Sleep is regulated by interacting systems in the brain. Two key principles are:
- Circadian rhythm (time-of-day control)
- Sleep homeostasis (need for sleep depending on prior wakefulness)
Circadian Rhythms and the “Body Clock”
Many sleep properties follow an approximately 24-hour rhythm. This is controlled by:
- An internal “clock” in the brain (in mammals located in the suprachiasmatic nucleus, SCN, of the hypothalamus).
- Inputs from environmental signals, primarily light entering the eyes.
Core features:
- The clock generates a daily pattern of:
- Sleep tendency
- Body temperature
- Hormone secretion (e.g., melatonin, cortisol)
- Light, especially in the morning, synchronizes (“entrains”) this internal clock to the external day-night cycle.
- Melatonin, produced mainly at night, contributes to signaling “biological night” to the body.
Other animals show circadian control of rest-activity cycles as well, even in constant darkness, indicating internal timing mechanisms.
Sleep Homeostasis
The longer an organism is awake, the stronger the tendency to fall asleep:
- Sleep pressure increases with wake time.
- This is partly reflected in accumulation of certain signaling molecules, for example adenosine in the brain.
- Deep, slow-wave sleep (N3) is more intense after prolonged wakefulness, suggesting it “pays back” sleep debt.
Together, circadian drive and homeostatic sleep pressure determine:
- When sleep is initiated
- How long it lasts
- How much deep slow-wave sleep occurs
Sleep Across the Animal Kingdom (Overview)
Sleep-like states have been identified in many groups:
- Mammals
- Show NREM- and REM-like states.
- Some species sleep a lot (e.g., bats), some very little (e.g., certain marine mammals).
- Birds
- Also exhibit NREM and REM sleep.
- Some birds can sleep with one brain hemisphere at a time (unihemispheric sleep), keeping the other side awake to watch for predators or continue flying.
- Reptiles, amphibians, fish, invertebrates
- Exhibit rest states with reduced responsiveness and characteristic brain or neural activity patterns.
- In some species, homeostatic regulation of these states and “rebound” after deprivation are seen, fulfilling key criteria for sleep.
These examples show that sleep, or sleep-like rest, is widespread and adapted to ecological needs (e.g., marine mammals coming up for air, migratory birds).
Sleep and Memory
Sleep plays a central role in information processing and storage:
- Stabilization of memories:
- Newly learned information is initially fragile.
- During sleep, brain networks supporting relevant memories are reactivated and strengthened (consolidation).
- Selection and reorganization:
- Important information can be preferentially stabilized.
- Connections can be reorganized, integrating new with old knowledge.
Different sleep stages are associated with different memory types:
- Slow-wave sleep (N3):
- Involved in consolidating facts and events (declarative memory).
- The hippocampus and cortex interact to “replay” experiences.
- REM sleep:
- Linked to procedural memory (skills, habits) and emotional memory.
- Neural networks relevant to emotions and motivation are active.
From a functional point of view, sleep can thus be seen as an offline mode of information processing: external inputs are reduced, allowing internal reprocessing of stored information.
Sleep Deprivation and Its Consequences
Lack of sufficient or regular sleep affects many aspects of nervous system function:
- Cognitive effects
- Reduced attention and concentration
- Slower reaction times
- Impaired learning and memory performance
- More errors and poorer decision making
- Emotional and social effects
- Increased irritability, mood swings, reduced stress tolerance
- Changes in emotional regulation and empathy
- Physiological and health-related effects
- Disturbed hormonal regulation (e.g., stress hormones, appetite-regulating hormones)
- Altered immune function
- Longer-term associations with metabolic and cardiovascular problems when chronic
Extreme or prolonged sleep deprivation in animals can be life-threatening, underlining the biological necessity of sleep.
Sleep Disorders (Overview)
Many variations in sleep are harmless or only mildly troublesome. However, some disorders strongly interfere with normal brain function and daytime performance. From the viewpoint of information processing, important groups include:
- Insomnia: difficulties falling or staying asleep; often leads to insufficient sleep quantity or poor sleep quality.
- Hypersomnia and narcolepsy: excessive sleepiness, sudden sleep attacks, or disturbances in sleep-wake control.
- Sleep-related breathing disorders (e.g., obstructive sleep apnea): repeated breathing interruptions during sleep, causing frequent awakenings and fragmented sleep.
- Parasomnias: unusual behaviors during sleep (e.g., sleepwalking, night terrors) that arise from partial arousal.
These disorders interfere with the normal architecture of sleep (distribution of sleep stages and cycles) and thus with its restorative and memory-supporting functions.
Sleep in Development and Aging
The pattern and function of sleep change across the lifespan:
- Before birth and in early infancy
- Very high total sleep time.
- Large proportion of REM-like sleep, thought to be important for brain development.
- Childhood and adolescence
- Deep N3 sleep is prominent.
- Circadian rhythms and sleep timing shift (e.g., tendency for later bedtime in adolescence).
- Adulthood
- Total sleep time stabilizes; sleep architecture is relatively stable but influenced by lifestyle and environment.
- Older age
- Less deep N3 sleep, more awakenings at night.
- Circadian rhythms may shift earlier.
- Fragmentation of sleep can affect memory and daytime cognitive performance.
These changes illustrate that sleep is closely coupled to developmental processes in the nervous system and to age-related changes in brain and body.
Adaptive Aspects of Sleep Timing and Behavior
When and how long an organism sleeps is shaped by evolution:
- Diurnal vs. nocturnal vs. crepuscular species
- Species adapt their active and rest phases to avoid predators, optimize foraging, and reduce competition.
- Sleep location and posture
- Choice of safe places (burrows, trees, hidden spots) reduces predation risk.
- Some animals sleep while standing (e.g., horses) or while floating in water.
- Flexible sleep strategies
- Migratory birds can reduce or fragment sleep during long flights.
- Marine mammals may sleep with one hemisphere to continue swimming and surfacing for air.
In all cases, sleep remains present, but its pattern is adapted to ecological constraints while still maintaining vital functions in information processing and health.
Summary
- Sleep is a biologically regulated, reversible state with distinct stages (NREM and REM), recurring in cycles.
- It is controlled by circadian clocks and homeostatic sleep pressure.
- Sleep is widespread across animals and essential for nervous system function.
- During sleep, the brain actively restores, reorganizes, and consolidates information, making sleep a critical phase of information processing and storage.
- Insufficient or disturbed sleep impairs cognition, emotion, and physiology, highlighting its central role in maintaining brain and body function.