Chapter 4. Consciousness and Sleep

4.3. Why do we sleep?

Given the central role that sleep plays in our lives and the number of adverse consequences that have been associated with sleep deprivation, one would think that we would have a clear understanding of why it is that we sleep. Unfortunately, this is not the case; however, researchers have proposed various different hypotheses to explain the function of sleep. Let’s take a look at some of them.

Adaptive Function of Sleep

One popular hypothesis of sleep incorporates the perspective of evolutionary psychology. 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. One hypothesis from this perspective argues that sleep is essential to restore resources that are expended during the day. Just as bears hibernate in the winter when resources are scarce, perhaps people sleep at night to reduce their energy expenditures. While this is an intuitive explanation of sleep, there is little research to support this explanation. In fact, it has been suggested that there is no reason to think that energetic demands could not be addressed with periods of rest and inactivity while we are awake (Frank, 2006; Rial et al., 2007), and some research has actually found a negative correlation between energetic demands and the amount of time spent sleeping (Capellini et al., 2008).

Another evolutionary hypothesis of sleep suggests that our sleep patterns evolved as an adaptive response to predatory risks, which increase in darkness. Thus we sleep in safe areas to reduce the chance of harm. Again, this is an intuitive and appealing explanation for why we sleep. Perhaps our ancestors spent extended periods of time asleep to reduce attention to themselves from potential predators. Comparative research indicates, however, that the relationship between predatory risk and sleep is very complex and equivocal. Some research suggests that species that face higher predatory risks sleep fewer hours than other species (Capellini et al., 2008), while other researchers suggest there is no relationship between the amount of time a given species spends in deep sleep and its predation risk (Lesku et al., 2006).

It is quite possible that sleep serves no single universally adaptive function, and different species have evolved different patterns of sleep in response to their unique evolutionary pressures. While we have discussed the negative outcomes associated with sleep deprivation, it should be pointed out that there are many benefits that are associated with having adequate amounts of sleep. A few such benefits listed by the National Sleep Foundation (n.d.) include maintaining healthy weight, lowering stress levels, improving mood, and increasing motor coordination, as well as better cognitive function and memory.

Cognitive Function of Sleep

Another theory regarding why we sleep involves the importance of sleep for cognitive function and memory formation (Rattenborg et al., 2007). Indeed, we know sleep deprivation results in disruptions in cognition (Brown, 2012), leading to difficulties in maintaining attention, making decisions, and recalling long-term memories. Moreover, these impairments become more severe as the amount of sleep deprivation increases (Alhola & Polo-Kantola, 2007). Furthermore, slow-wave sleep seems essential for effective memory formation (Stickgold, 2005). Having periods of slow wave sleep after learning a new task improves subsequent performance on that task (Huberet al., 2004). Understanding the impact of sleep on cognitive function should help you understand that cramming all night for a test is often not effective and can even prove counterproductive.

Link to Learning

Watch this brief video that gives sleep tips for college students to learn more.

Getting the optimal amount of sleep has also been associated with other cognitive benefits, including better creative thinking (Cai et al., 2009; Wagner et al., 2004) and language learning (Fenn et al., 2003; Gómez et al., 2006), and improved ability in making inferential judgments (Ellenbogen et al., 2007). Effective emotion processing is also influenced by certain aspects of sleep (Walker, 2009).

Link to Learning

Watch this brief video about the relationship between sleep and memory to learn more.


How people think about dreams varies across different cultures and the psychology of dreaming in the Western world has also changed over time. By the late 19th century, Austrian psychiatrist Sigmund Freud had become convinced that dreams represented an opportunity to gain access to the unconscious. By analyzing dreams, Freud thought people could increase self-awareness and gain valuable insight to help them deal with the problems they faced in their lives. Freud made distinctions between the manifest and the latent content of dreams. Manifest content is the actual content, or storyline, of a dream. Latent content, on the other hand, refers to the hidden meaning of a dream. For instance, if a woman dreams about being chased by a snake, Freud might have argued that this represents the woman’s fear of sexual intimacy, with the snake serving as a symbol of a man’s penis.

Freud was not the only Western theorist to focus on the content of dreams. The 20th century Swiss psychiatrist, Carl Jung, believed that dreams allowed us to tap into the collective unconscious. The collective unconscious, as described by Jung, is a theoretical repository of information which he believed was shared by everyone. According to Jung, certain symbols in dreams reflect universal archetypes, and their meanings are similar for all people regardless of culture or location.

The sleep and dream researcher, Rosalind Cartwright, however, believes that dreams simply reflect life events that are important to the dreamer. Cartwright’s ideas about dreaming have found empirical support. For example, she and her colleagues published a study in which women going through divorce were asked several times over a five-month period to report the degree to which their former spouses were on their minds. These same women were awakened during REM sleep in order to provide a detailed account of their dream content. There was a significant positive correlation between the degree to which women thought about their former spouses while they were awake and the number of times their former spouses appeared in their dreams (Cartwright et al. 2006).

Allan Hobson, a neuroscientist, is credited for developing the activation-synthesis theory of dreaming. Early versions of this theory proposed that dreams were not the meaning-filled representations of angst proposed by Freud and others, but were rather the result of our brain attempting to make sense of (“synthesize”) the neural activity (“activation”) that was happening during REM sleep. Recent adaptations (e.g., Hobson, 2002) continue to update the theory based on accumulating evidence. For example, Hobson (2009) suggests that dreaming may represent a state of protoconsciousness. In other words, dreaming involves constructing a virtual reality in our heads based on the activation that occurs in different parts of our brain during REM sleep. For example, when we experience random activation of the motor cortex we dream that we are moving and activation of the vestibular system might lead us to dream that we are falling or flying. Hobson suggests that learning to interpret our brain activity while we are asleep, might serve as a model so that we can use similar interpretive processes when we are conscious. Among a variety of neurobiological evidence, Hobson cites research on lucid dreams as an opportunity to better understand dreaming in general. In a lucid dream, a person becomes aware of the fact that they are dreaming, and as such, they can control the dream’s content (LaBerge, 1990). Other research (Horikawa et al., 2013) has uncovered new techniques by which researchers may effectively detect and classify the visual images that occur during dreaming by using fMRI to measure brain activity. This may open the way for additional research in this area.

There are considerable cultural variations in how people view their dreams. Whites are more likely to dismiss them as unimportant compared to African Americans, people from Africa and Latinx people. In many non-White cultures, dreams are considered an important part of consciousness and are central to peoples’ lives. In collectivistic cultures that embrace spirituality, religion, and magical realism, dreams are often a way to connect with the dead. Dreams often provide messages either for the dreamer or for someone else, which is consistent with the fact that people in collectivistic cultures typically have strong ties with their community. Messages in dreams vary in content, for example, they might give reassurance that the deceased is at peace, or they might provide guidance, approval, or inspiration for their loved ones for their life choices. Alternatively, messages may be prophetic or premonitory in nature. It is also usual for people who value dreams to share them with other members of their community (Comas-Diaz, 2006; Nwoye, 2017; Shafton, 2002).


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Introduction to Psychology (A critical approach) Copyright © 2021 by Jill Grose-Fifer; Rose M. Spielman; Kathryn Dumper; William Jenkins; Arlene Lacombe; Marilyn Lovett; and Marion Perlmutter is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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