Animals are typically active during the day (diurnal) or night (nocturnal), and due to these adaptations, they are more efficient during their selected time of activity. For example, we have poor night vision, so we function best with the adequate light during the daytime. Nocturnal mice sneak around best in the dark, avoiding predators. It makes logical sense to hole up somewhere safe when it is not your preferred time of day, and sleep helps with that conformity. 

Niche adaptation

Sleep seems to help us ward of illness, as well as allow time for the brain to rid itself of waste products that build up over time. This brain cleanup is due to a faster flow of cerebrospinal fluid during sleep, speeding up toxin removal by glia. 

Body restoration

According to the energy conservation hypothesis, a key function of sleep is to reduce an organism's energy expenditure. This perspective, rooted in evolutionary psychology, suggests that by lowering metabolic activity during sleep, individuals can conserve resources that were used up during the day. This behavior is thought to be an adaptation that enhances survival by reducing the need to constantly search for scarce resources.

Energy Conservation Hypothesis of Sleep

This evolutionary hypothesis proposes that sleep is an adaptive behavior for protection against predators. The theory suggests that sleep patterns evolved to keep organisms inactive and concealed during times of increased vulnerability, such as in darkness, thereby reducing the risk of harm. However, the evidence for this is complex and inconclusive, as comparative research has not found a clear, consistent relationship between a species' predation risk and its sleep duration.

Predator Avoidance Hypothesis of Sleep

Sleep is believed to be essential for memory consolidation, the process of stabilizing a memory trace after its initial acquisition. Different sleep stages may consolidate different memory types; for instance, NREM sleep is linked to declarative memories, while REM sleep is associated with nondeclarative memories. Specifically, slow-wave sleep is considered crucial for effective memory formation, potentially through the strengthening and pruning of synapses.

Memory Consolidation During Sleep

A college student pulls several 'all-nighters' to study for final exams. Despite the extra study time, they perform poorly on the exams, feel physically exhausted, and catch a cold immediately after the exam period. Analyze this student's experience by connecting each of their three specific outcomes (poor exam performance, exhaustion, illness) to a distinct proposed biological function of sleep.

Analyzing the Consequences of Sleep Deprivation

The exact reasons we sleep are unclear and hotly debated, but there are four main explanations:

- Energy conservation
- Niche adaptation
- Body restoration
- Memory consolidation

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University of Rochester

- Sleep stages
- Sleep patterns over a lifetime
- Sleep onset
- Sleep regularity
- Biological functions of sleep
- Neural systems underlying sleep
- Sleep deprivation
- General anesthetics
- Sleep disorders
- Tracking Information During Sleep
- Sleep-wake cycle

Sleeping and waking

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

Throughout sleep, there are different stages characterized by the presence or absence of types of brain waves. Typically, we cycle through these stages 4-5 times a night, sleeping on average 7-8 hours. These stages are broken down into two categories:

- Non-REM
- REM

Sleep Stages

The partial or total prevention of sleep. Side effects could include hallucinations, but most commonly witnessed effects are increases irritability, decreased ability to concentrate, and episodes of disorientation. Cognitive deficits are noticeable. Prolonged sleep deprivation compromises the immune system in mammals and can lead to death.

Sleep deprivation

Biological functions of sleep

Sleep library

Sleep regularity refers to whether a person sleeps and wakes at a consistent time. 

Sleep Regularity

Sleep onset is when a person transitions from a state of wakefulness and falls asleep.

Sleep Onset

A drug that renders an individual unconscious. Examples include barbiturates and anesthetic gases. They produce slow waves in the EEG that resemble the ones seen in slow wave sleep, suggesting that they tap into existing brain networks that promote sleep. Almost all are non-competitive agonists at GABA(A) receptors, boosting GABA’s inhibitory effect on neurons.

General Anesthetics

This study investigated sleepers' abilities to filter and allocate their attention towards the most relevant source of information when presented with two streams of dialogue. As of the study, there had been no evidence that the sleeping brain could covertly select which piece(s) of information to prioritize when exposed to multiple pieces of information. This study aims to see if the sleeping brain will amplify meaningful speech compared to irregular signals.

"Sleepers Track Informative Speech in a Multi-talker Environment" Legendre et al. (2019)

Sleep Library

Sleep-Wake Disorders are defined using a multidimensional approach due to the expectations for co-existing or comorbid diagnosis in both mental and neurological health. They are largely characterized by individual’s interpretation of having dissatisfying sleep quality, timing, or amount of sleep. These complaints are often expressed alongside issues of distress or impairment during expected daytime activities. 

Sleep-Wake Disorders

The sleep-wake cycle is a fundamental circadian rhythm that is internally regulated by the body's biological clock. This process governs the daily pattern of sleeping and waking, ensuring it is synchronized with the 24-hour light-dark cycle of the environment.

Sleep-Wake Cycle

Sleep-wake cycles are regulated by several brain areas working together. The thalamus, hypothalamus, and pons are key regions in this coordination: the hypothalamus includes the SCN, described as the body's biological clock, other hypothalamic nuclei work with the thalamus to regulate slow-wave sleep, and the pons helps regulate rapid eye movement (REM) sleep.

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