Sleep is a vital part of our routine and many of us can never seem to get enough of it. It is defined as a state of immobility with greatly reduced responsiveness that can be rapidly reversed. However, the reason we sleep has yet to be elucidated by the scientific community. One possible explanation, the synaptic homeostasis hypothesis, states that sleep plays a role in the regulation of synaptic weight, which is the strength of connection between two neurons in the brain. This hypothesis suggests that connection strength between neurons increases while we are awake and peaks right before we sleep. This is followed by a decrease in strength to a baseline level during sleep. The process can be thought of as a cycle of “heating up and cooling down” in the brain to maintain homeostasis, allowing the brain to save energy, prevent synaptic overload, facilitate learning, form new memories.
Evidently, brain activity does not cease during sleep. In the 1950s and 60s, the regular cyclic alternation between rapid eye movement (REM) and non-REM (NREM) during sleep was discovered. REM sleep is associated with vivid dreaming, which indicates that the brain is highly active during sleep.3 However, it was also noted that during REM sleep, sensory inputs and motor outputs are “shut off .” NREM sleep is characterized by a lack of dreams and a complete loss of consciousness, but some brain regions remain significantly active. These discoveries indicate that sleep is a “reorganization,” rather than a cessation, of neuronal activity.
The cycle of NREM and REM sleep seems to be conserved in almost all mammals, suggesting that sleep may have a universally significant function. In addition, all mammals and even individual humans differ in the amount and nature of their sleep, which are dependent with genetics, environmental factors, and diet.1 Interestingly, some aquatic mammals, such as dolphins, have adapted a unique way of sleeping called unihemispheric sleep, in which they “shut off ” only one hemisphere of their brain at a time. The study of sleep in multiple species further supports the theories that sleep saves energy, keeps species from being inactive at inopportune times, and differs qualitatively across individuals and species.
So what happens if we don’t sleep? One study found that a night of sleep resulted in a 20% increase in motor speed without loss of accuracy. Furthermore, a significant positive correlation was noted between the amount of NREM sleep and cognitive performance the following day. Many studies show that sleep deprivation a negative impact on mood, cognitive performance, and motor function. Lack of sleep also results in daytime performance deficits, which lead to significant social, financial, and human costs. Accidents related to sleep deprivation have been estimated to have an annual economic impact of $43 billion to $56 billion. Notably, lack of sleep has been shown to have similar impairments to those induced by alcohol consumption at or above the legal limit. Overall, sleep deprivation has been linked with reduced cognitive performance, attention deficits, weaker immune responses, high blood pressure, and decline in normal human function.
Written by Daniel Diatlov
References may be found in the journal.