Three Key Takeaways:
1. Sleep Patterns and Environmental Factors: Sleep serves multiple functions (eg, energy conservation, thermoregulation, and memory consolidation) and sleep patterns can change over time due to environmental factors, influencing rapid eye movement (REM) sleep.
2. Hunter-Gatherer Sleep Patterns: Contrary to previous assumptions, these groups rarely slept from dusk to dawn, but they presented lower rates of insomnia and better health outcomes.
3. REM and its Thermoregulatory Function: REM sleep may have initially evolved to serve a thermoregulatory function in the brain, helping to maintain brainstem function during sleep in mammals with relatively low body temperatures and metabolic rates.
Sleep has multiple functions including energy conservation, thermoregulation, and memory consolidation. However, sleep patterns can change over time because of environmental factors, which subsequently influence rapid eye movement (REM) sleep. In a review published in Lancet Neural, the author examines the associations between sleep duration, health, and cognition related to modern industrial environments. Further, the author discusses the possible function of REM and non-REM sleep across various species.
The author discussed a previous study that evaluated the sleep cycles of 2 tribal groups living in Africa (Namibia and Tanzania) and 1 living in South America (Bolivia). All 3 groups were reported to have minimal shelter, no electricity, and were self-sufficient, with little contact with industrial societies.
Unlike previous assumptions that people living a hunter-gatherer lifestyle sleep from dusk to dawn, the individuals from the 3 groups rarely went to sleep at sunset and instead, sleep onset occurred approximately 3.2 hours after sunset. Further, the individuals did not regularly awaken during the night or sleep during the day and sleep onset was found to be more variable than sleep offset.
The study had found that less than 2% of hunter-gathers in Namibia and Bolivia had reported or presented signs of insomnia—which is significantly lower than the 10% to 30% rate of insomnia present in some industrial societies—and suggests that widespread insomnia is not a natural characteristic of humans. The 6- to 8-hour sleep duration the 3 groups presented, compared to the 7- to 8-hour sleep duration in industrial societies, is associated with better health. For example, the Tsimane group in Bolivia had the lowest rate of coronary artery disease. In addition, there was no evidence to suggest that Alzheimer disease is a common occurrence.
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According to the current study author, sleep is a maladaptive state due to its ability to increase the vulnerability of animals to predation and conflicts with eating, reproducing, and protecting offspring; however, the balance between energy acquisition and energy expenditure is key in determining evolutionary fitness. Energy expenditure is decreased by periods of inactivity such as hibernation, extended periods of dormancy, and sleep, and these states increase evolutionary fitness. The study author notes nightly torpor—a state of significantly reduced body temperature after sleep onset in animals—as an example due to it being interpreted as an energy-saving adaptation.
In addition, REM sleep follows non-REM sleep in most mammals. Because REM sleep is a state of very high metabolic activity similar to waking, toxin clearance in non-REM sleep is undone by the re-creation of toxins in REM sleep. The author notes that another potential explanation of sleep function is the excess synapses that are removed during non-REM sleep, which is a process specific for brain region and age. Synapse removal and toxin drainage may occur during sleep; however, neither explain the variation in sleep (eg, duration of sleep, qualities of human sleep versus other mammals) across different species, according to the author.
Sleep duration varies across mammals and most—humans included—sleep for longer periods of time and experience deeper sleep when they are younger. Further, there is no clear correlation between the size of the brain or brain-to-bodyweight ratio and sleep duration; however, an inverse correlation between body mass and daily sleep duration was reported in early studies of sleep phylogeny.
REM sleep may have initially evolved to maintain brainstem function during sleep in mammals that had relatively low body temperatures and metabolic rates. In addition, REM sleep was thought of as a thermostatically controlled brain-heating mechanism which is triggered by the temperature reduction linked to the reduced metabolism, and the decrease in energy consumption in non-REM sleep. REM sleep ends after the amount of REM required to raise brain temperature to close to the waking temperature of the body has occurred.
The study author notes that prior data suggest REM sleep has a thermoregulatory role in the brain that is triggered by the reduction of brainstem temperature during non-REM sleep and stops once the target temperature is achieved. Body thermoregulation is maintained during non-REM sleep, although it is at a lower temperature than during waking. Further, body temperature either increases or decreases depending on the environmental temperature. According to the author, the thermoregulatory changes that occur during REM sleep serves 3 functions: heating the brain keeps it metabolically functional and able to awaken quickly, reducing or eliminating activity in the muscles of the body preserves energy, and the reduced muscle tone prevents dream-enacting behaviors that are generated when REM sleep is damaged, resulting in sleep behavior disorders.
Further, the study author discusses the sleep patterns of marine mammals, nothing that they have evolved unique sleep patterns that allow them to sleep while swimming. Because some of these animals have larger brains compared to humans, they can produce unihemispheric slow waves during sleep, known as unihemispheric sleep or half brain sleep. Unihemispheric sleep may not cool that brain stem, unlike bilateral non-REM sleep, therefore, the warming of the brainstem with REM sleep is not needed.
Sleep is a highly adaptive brain function due to its ability to reduce energy expenditure. Insomnia, which is common in humans living in industrial society, is potentially due to the removal of natural temperate and light cycles from a modernized environment.
According to the study author, this demonstrates that environmental variables may influence the sleep-regulating neurons, but further research is needed to confirm this association. In addition, body and pontine brainstem temperature recordings are needed to be performed during sleep recordings to understand the link between sleep and thermoregulation in animals.
Siegel JM. Sleep function: an evolutionary perspective. Lancet Neurol. 2022;21(10):937-946. doi:10.1016/S1474-4422(22)00210-1