A major factor affecting sleep duration varies among animals, is how great the threat of being eaten is. There is considerable variation in how long and deeply animals sleep. Giraffes will only typically take two to four hours of sleep per day and mostly in short intervals while they are standing up. Bats may be asleep for twenty hours per day, hanging upside down. Dolphins and whales have unihemispheric sleep, which means that only One side of the brain is sleeping and the other side stays awake to allow them to continue to surface to breathe and observe for threats. All of this variety demonstrates that there is no single way to sleep, and the reason for all of this variability is based on several different variables.
Predator risk is probably the biggest variable when considering what determines the differences in the amount and quality of sleep. Most predators feed upon prey that has lower predator risk. For example, a lion does not need to be as vigilant as a deer because lions are at the top of the food chain. In addition, animals that have greater Metabolic needs generally need more restorative sleep. Body size and dietary requirements also contribute to differences in the amount and quality of sleep.
The human brain is very complex and utilizes a lot of energy
The average adult human brain weighs approximately 1.5 kilograms or 3.3 pounds, however, uses up to twenty percent of total body energy at rest. Energy utilization by the brain during sleep differs from energy use during waking periods.
In addition to providing for basic physiological needs, the human brain supports higher order cognitive activities such as thinking abstractly, using language, solving problems complexly, and engaging in intricate social behaviors. These higher-order cognitive processes require continuous support through both the formation and consolidation of memory and “housekeeping,” both of which occur largely during sleep. Additionally, the brain continues to process large quantities of information each day, learn new things, and adapt to changing conditions, all of which demands a significant and sustained period of rest and restoration.
Sleep performs a number of important roles for humans including facilitating the consolidation of memory. As people sleep, their brains continue to process and solidify memories that were formed during the previous waking period. Through NREM (Non-Rapid Eye Movement) sleep, the brain replays and transmits information from the hippocampus (temporary storage) to the neocortex (permanent storage). Rem (rapid eye movement) sleep plays a critical role in the processing of emotional memory as well as integrating new information with pre-existing knowledge. Research performed by dr. Matthew Walker demonstrates how severely the capacity for learning and remembering is impacted after only partially deprived of sleep.
Brain plasticity and development
As discussed earlier, humans have a relatively long developmental window, extending well beyond that seen in most other species. From infancy through adolescence, humans’ brains are continually developing through rapid networking and de-networking. Sleep is essential to this process. Infants, who have a relatively large proportion of their day spent sleeping, (which can range from fourteen to seventeen hours depending on age) have a large component of this sleep classified as rem sleep, which is critical for brain development. The fact that children develop over such a long period and require significant amounts of sleep further suggests that quality rest provides a foundation for the complex cognitive architecture seen in humans.
The consequences of sleep deprivation in humans
While sleep deprivation causes problems for all living organisms to varying degrees, its effects on human cognitive functioning, emotional states, and physical health are quite marked and extensively documented.
Immediate consequences of sleep deprivation include decreased attentiveness, diminished ability to solve problems creatively, delayed reaction times, and increased irritation. Driving after being deprived of sleep is equally hazardous as driving drunk. Prolonged sleep deprivation increases susceptibility to serious health issues like obesity, type ii diabetes, cardiovascular disease, etc., as well as weakening your immune system. Furthermore, it affects mental health by raising the possibility of developing depression and/or anxiety.
Consider a rat. Rats are frequently used in research regarding sleep deprivation due to their willingness to perform various tasks for rewards. Both rats and humans experience cognitive impairment from prolonged deprivation of sleep. Since rats’ brains are anatomically different from humans’, they would exhibit different types of cognitive dysfunction than humans would. Although rats may appear less cognitively impaired than humans in terms of immediate task performance, they still demonstrate noticeable deficits. A study published in Science in 2013 found that researchers discovered a waste-clearance system in the brains of mice called the glymphatic system. During sleep, this system was shown to clear neurotoxic waste products that accumulated in the brain during wakeful periods. If rats or humans do not receive adequate sleep, then this system will not effectively remove neurotoxins and could lead to neurodegenerative diseases over time.
Social Structures & vulnerability
Humans have incredibly complex social relationships and depend greatly on cooperative behavior. Therefore, sleep deprivation could seriously disrupt our ability to recognize the feelings of others, control our own emotions and make rational decisions about others. Thus, we are not only individually vulnerable but collectively as well. An example would be an accident caused by a sleep-deprived surgeon or pilot. This collective vulnerability due to individual sleep debt contributes to why sufficient sleep is so vital to humans in today’s society.
Evolutionary trade-offs
Considering evolutionarily speaking, the necessity for extensive hours of human sleep is puzzling. Why would an organism put itself in an unconscious condition for nearly One-third of its lifetime? How did this come to pass? One possible explanation is that the advantages provided by neural repair during Non-Rapid Eye Movement (NREM) sleep; memory consolidation during slow-wave-sleep (sws); and cognitive optimization during rapid-eye-movement (rem) sleep outweighed the disadvantages provided by potential predation during extended periods of unconsciousness. Early humans likely slept in close proximity with others in protected environments reducing the risks of predation. Additionally, early humans had a longer dependent child-rearing period followed by a long developmental window for the maturation of the brain, highlighting the requirement for secure extended timeframes for vulnerable young.
All told, although humans share an elementary biological requirement for sleep with all other animals, the reasons we go to bed and the implications of getting enough rest or not are tightly connected with our complex cognition and social nature. Therefore, we do not require more hours of sleep than *any* other animals but rather the depth of consequence that occurs when we fall short in providing the required restorative conditions for our complex brain is unlike anything else seen in nature.
Why do humans need so much rem sleep?
Humans spend a significant portion of their time in rem (rapid eye movement) sleep relative to other stages of sleep. Specifically, infants spend a larger percentage of their time in rem sleep than any other stage; whereas older children and adults spend progressively smaller percentages in rem relative to NREM (Non-Rapid Eye Movement) stages. The primary reason we spend so much time in rem sleep is that this stage appears to be critical for regulating emotionality; processing emotionally charged memories; integrating new information into existing knowledge bases; supporting continued growth/developmental changes in the cerebral cortex; and allowing for the continued practice/consolidation of skills learned during wakefulness.
Do all animals dream?
Unfortunately, it is impossible to know whether all animals dream as dreaming is inherently subjective. Nonetheless, since rem sleep; the stage associated with dreaming in humans; exists throughout much of the mammalian world; and also extends into avian species; it is reasonable to assume that many animals are capable of experiencing some form of dreaming or brain activity akin to dreaming.
Studies examining brain activity during rem sleep reveal patterns of activity during rem that resemble those observed during wakefulness; suggesting that some form of dreaming or consciousness may exist in many species.
Can humans adapt to less sleep over time?
Although some people claim to be able to survive with minimal amounts of sleep; scientific data unequivocally indicates that chronic reductions in sleep result in measurable declines in cognitive performance and overall health status. The body may adapt; meaning you become accustomed to feeling less tired; however measurable decrements in cognitive function persist regardless of adaptation. According to recommendations made by The National Sleep Foundation, most adults should obtain between seven and nine hours of sleep per night.
What happens if humans don’t get enough sleep?
Acute reductions in sleep cause declines in attentional capabilities; memory retention/recall; decision-making ability; increased irritability; etc… Furthermore; chronic reductions in sleep have been causally linked to multiple serious medical problems such as obesity; diabetes; heart disease; compromised immune systems; etc… Additionally; chronic reductions in sleep increase the probability of having automobile-related accidents; as well as psychiatric illnesses such as depression.
Humans are unique in requiring a certain degree and quantity of quality sleep necessary for our elaborate brains; demanding social relationships; etc.. It is not merely about giving ourselves a break physically – but essentially about restoring/reorganizing/repairing our exceptionally complex neural machinery that enables us to think; feel; communicate/socially interact as we do.
Sources
- Why Do We Sleep? — Sleep Foundation
- The Role of Sleep in Memory Consolidation — NCBI (National Center for Biotechnology Information)
- Sleep Drives Metabolite Clearance from the Adult Brain — Science
- The social costs of sleep deprivation — PNAS (Proceedings of the National Academy of Sciences)
- Interactions Between Sleep, Learning, and Memory — American Psychological Association
- Understanding sleep — Harvard Health Publishing
