Sleep: Learn It 3—Why Do We Sleep?

Why Do We Sleep?

We (humans) spend approximately one-third of our lives sleeping. Some animals never sleep (e.g., several fish and amphibian species); other animals can go extended periods of time without sleep without any apparent negative consequences (e.g., giraffes); yet some animals (e.g., rats) die after two weeks of sleep deprivation (Siegel, 2008). Why do we devote so much time to sleep? Is it absolutely essential that we sleep?

Given how important sleep is, you might expect a single clear answer for why we sleep—but the science shows multiple useful theories.

Adaptive Function of Sleep

From an evolutionary psychology perspective, sleep may have developed because it offered survival advantages.

Evolutionary psychology is a discipline that studies how universal patterns of behavior and cognitive processes have evolved over time as a result of natural selection. Variations and adaptations in cognition and behavior make individuals more or less successful in reproducing and passing their genes to their offspring.

1. Energy Conservation Hypothesis

One hypothesis proposes that sleep allows organisms to conserve energy and restore resources expended during wakefulness. This would make sense for species that face limited food availability or harsh environmental conditions.

However, studies show mixed results—species with higher metabolic rates often sleep less, not more (Capellini et al., 2008; Gavrilov et al., 2023).[1] Thus, energy restoration alone may not fully explain why we sleep.

Recent research using cross-species modeling suggests that sleep’s energy benefits may depend on how efficiently the brain clears waste through the glymphatic system, which increases activity during deep sleep.[2]

2. Predator-Avoidance Hypothesis

Another hypothesis suggests that sleep evolved as an adaptive response to reduce risk of predation. Staying still and hidden at night could have increased the chances of survival for early humans and animals.

Yet, comparative data indicate the relationship between predation risk and sleep duration is inconsistent: some species facing high predation risk sleep less, while others show no difference (Lesku et al., 2006).

3. Species-Specific Adaptations

Current research suggests that different species evolved distinct sleep patterns based on ecological needs rather than a single universal function (Lesku et al., 2006; Capellini et al., 2008).

Recent work in comparative neurobiology highlights that both REM and non-REM sleep serve diverse purposes across species, such as memory processing or immune repair (Yamazaki et al., 2020).[3]

Cognitive and Brain-Function Hypotheses

Beyond evolutionary advantages, sleep plays a vital role in brain health, cognitive performance, and memory consolidation.

1. Memory and Learning

Sleep helps the brain process, stabilize, and integrate new information. Both slow-wave (deep, non-REM) and REM sleep appear to support different types of memory. Meta-analyses show that sleep loss significantly impairs working memory, attention, and executive functioning (Lim & Dinges, 2010; Lowe et al., 2017).[4]

Research also shows that post-learning sleep enhances task performance and retention (Huber et al., 2004; Stickgold, 2005).

2. Creativity and Problem-Solving

Sleep promotes creative thinking and insight. Studies show participants perform better on creative problem-solving tasks after REM-rich sleep (Cai et al., 2009; Wagner et al., 2004).

3. Emotional Regulation

Modern neuroscience highlights sleep’s role in regulating mood and emotional reactivity. Sleep deprivation increases amygdala activation and reduces prefrontal control, leading to stronger emotional responses (Walker & Van Der Helm, 2009). Studies confirm that adequate sleep improves emotion recognition, resilience, and social functioning (Ben Simon & Walker, 2018; Palmer & Alfano, 2017).

4. Brain and Physical Health

Sleep allows for cellular repair, immune function, and detoxification. During deep sleep, the glymphatic system clears waste products such as beta-amyloid, a protein linked to Alzheimer’s disease. Adequate sleep also supports metabolic health and hormone regulation.

In 2022, the American Heart Association formally added sleep duration to its “Life’s Essential 8” metrics for cardiovascular health, underscoring its role in preventing disease.[5]

Watch this video to learn more about the function of sleep and the harmful effects of sleep deprivation.

You can view the transcript for “What would happen if you didn’t sleep? – Claudia Aguirre” here (opens in new window).

Sleep Quality, Duration, and Regularity

Recent research emphasizes that how well and how consistently you sleep matters just as much as how long.

  • Sleep regularity—maintaining consistent bedtimes and wake times—strongly correlates with better mental and physical health outcomes, independent of total sleep duration (Sletten et al., 2023).[6]
  • Sleep quality and continuity are linked to cognitive performance, with sleep disturbances predicting worse cognitive function in subsequent years (Björnsdóttir et al., 2022).[7]
  • Public health data show worsening sleep trends globally—37% of U.S. adults reported sleeping worse in 2023 than in prior years (Sleep Foundation, 2023). [8]

Neuroscientists are exploring “circadian regularity” as a health metric—showing that people who maintain consistent sleep-wake timing have more stable mood, better metabolism, and improved immune response (Tomatsu et al., 2025).[9]

Learn about the connection between memory and sleep in the following clip:
You can view the transcript for “The Connection between Memory and Sleep – Science Nation” here (opens in new window).
  1. Gavrilov, V. M., Golubeva, T. B., & Bushuev, A. V. (2023). Metabolic rate, sleep duration, and body temperature in evolution of mammals and birds: The influence of geological time of principal groups divergence. Biology, 12(5), 679. ↵
  2. Semyachkina-Glushkovskaya, O., Fedosov, I., Penzel, T., Li, D., Yu, T., Telnova, V., Kaybeleva, E., Saranceva, E., Terskov, A., Khorovodov, A., Blokhina, I., Kurths, J., & Zhu, D. (2023). Brain Waste Removal System and Sleep: Photobiomodulation as an Innovative Strategy for Night Therapy of Brain Diseases. International journal of molecular sciences, 24(4), 3221. https://doi.org/10.3390/ijms24043221 ↵
  3. Yamazaki, R., Toda, H., Libourel, P. A., Hayashi, Y., Vogt, K. E., & Sakurai, T. (2020). Evolutionary origin of distinct NREM and REM sleep. Frontiers in Psychology, 11, 567618. ↵
  4. Lowe, C. J., Safati, A., & Hall, P. A. (2017). The neurocognitive consequences of sleep restriction: A meta-analytic review. Neuroscience & Biobehavioral Reviews, 80, 586-604 ↵
  5. Dinh, V. T., Hosalli, R., Mullachery, P. H., Aggarwal, B., German, C. A., & Makarem, N. (2024). Enhancing the Cardiovascular Health Construct With a Psychological Health Metric for Predicting Mortality Risk. JACC. Advances, 3(8), 101112. https://doi.org/10.1016/j.jacadv.2024.101112 ↵
  6. Sletten, T. L., Weaver, M. D., Foster, R. G., Gozal, D., Klerman, E. B., Rajaratnam, S. M. W., Roenneberg, T., Takahashi, J. S., Turek, F. W., Vitiello, M. V., Young, M. W., & Czeisler, C. A. (2023). The importance of sleep regularity: A consensus statement of the National Sleep Foundation sleep timing and variability panel. Sleep Health, 9(6), 801-820. https://doi.org/10.1016/j.sleh.2023.07.016 ↵
  7. Björnsdóttir, E., Jansson-Fröjmark, M., Lundgren, J., & Bohman, B. (2022). Sleep disturbance predicts worse cognitive performance in subsequent years: A longitudinal population-based cohort study. Sleep Medicine, 100, 1-8. https://doi.org/10.1016/j.sleep.2022.11.017 ↵
  8. Sleep Foundation. (2023, December 15). The year in sleep: 37% of us slept worse in 2023. https://www.sleepfoundation.org/sleep-news/the-year-in-sleep-in-review ↵
  9. Tomatsu, S., Abbott, S. M., & Attarian, H. (2025). Clinical chronobiology: Circadian rhythms in health and disease. Seminars in Neurology, 45(3), 317-332. https://doi.org/10.1055/a-2538-3259 ↵