Sleep knits up the ravelled sleave of care; the death of each day’s life, sore labour’s bath; balm of hurt minds, great nature’s second course; chief nourisher in life’s feast. ~ William Shakespeare, in MacBeth (1606)
Sleep has hoary evolutionary roots. Plants sleep. Tree branches droop during the night. Keeping up internal water pressure (turgor pressure) takes energy, so trees rest after dark.
During the day, trees hoist their branches and angle their leaves upwards to catch more sunlight and reduce shading. At night, this effort serves no purpose.
All animals – invertebrates as well as vertebrates – have inactive respite. Rest as a balance to activity is a simple rhythmic system that serves an essential metabolic need. Sleep is one of the many biological processes under sway of circadian rhythm, primarily by exposure to light.
Many marine organisms have lunar clocks to keep track of the tides. Moonbeams also affect landlubbers.
Human sleep patterns click to a lunar tick. It takes longer to fall asleep as the moon waxes into a cheese-colored orb. People sleep ~20 minutes less and less deeply when the moon is full.
Roundworms cyclically enter a sleep-like state of repose. Insects sleep. They may well dream of hopes and fears as we do: of juicy treats, lascivious sex, and threatening harassments while simply trying to make it through the day. Flies deprived of sleep become bleary.
For safety rodents take short naps and are easily aroused. Lizards, birds, and aquatic mammals go one better: half the brain sleeps while the other half stays awake. Birds that fly non-stop for days can sleep in flight.
Different mammals sleep significantly different percentages of their lives away. Bats sleep 18–20 hours a day, while giraffes get by with only 3–4 hours per day. There can be major discrepancies in sleep durations between closely related species.
Sleep patterns vary partly owing to circadian clock durations. Flying squirrels have a 23-hour day; monkeys 24.5; humans 25. The body clock can vary among individuals.
Carnivores get a lot of sleep; omnivores less.
Herbivores need the least sleep, but many spend much of their waking time drowsy. There is a correlation between herbivore size, metabolism, and sleep duration. Large herbivores live slower lives and so need less sleep.
More generally, large animals have a longer sleep cycle than smaller ones. Smaller mammals tend to sleep more during the day, and for short snoozes: perhaps because they need to eat several times a day.
Mammals that are less developed at birth, including rats, cats, and primates, tend to spend more time dreaming than those born with well-developed regulatory systems, such as ungulates.
The durations that primates tend to sleep generally corresponds to body size, along with other factors, including the average number in a group. Humans are an outlier in sleep: exceptionally short and deep.
Monkeys sleep in the trees. Napping on a branch is anything but easy. A monkey may get roused by a breeze or the jostling of one’s fellows. But better light sleep than to become a snack for whatever prowls the trees at night, looking for prey.
Apes take great care in choosing their sleeping spot, and in the materials used in building a nest for the night. An ape aims for the best night’s rest it can get. Unsurprisingly, cognitive performance is impaired if an ape is not well-rested.
Part of hominin evolution was to garner more efficient sleep: to get to higher-quality sleep quicker, and so require less of it. Humans more quickly fall soundly asleep and spend more of it dreaming (22%) than other primates.
A good night’s sleep cannot be had in a strange place. In unfamiliar surroundings, the brain’s left hemisphere stays active while the rest of the brain slumbers. Thus, the first night in a hotel is not especially restful, regardless of fatigue.
While human sleep is unusual for primates, it is not so special among mammals. The platypus spends half of its slumber dreaming away.
Hibernation is a state of torpor, not sleep. Hibernation markedly moderates the need for sleep but does not banish it. Hibernation is interrupted multiple times so that an animal can get some sleep. The hypothermia of hibernation is interrupted for the euthermia of sleep. This energetically enigmatic transition highlights the importance of sleep.
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Human sleep comprises 2 states of consciousness: dreaming and non-dreaming. Sleep cycles between the 2 states, typically 4 to 5 times a night. The duration of these cycles usually varies during the night but is typically ~90 minutes.
Sleep varies throughout life. Dreaming is essential to infant mind-brain development, notably executive functioning that develops between ages 1–6.
From early childhood less time is spent dreaming until, reaching adulthood, it becomes a small fraction of daily life. Non-dreaming sleep also lessens with age.
The average person spends 25 years of life asleep. Most adults function best with 7–9 hours.
It is an irony of post-industrial societies that so-called prosperity casts so many on the shores of insufficient sleep. 1/3rd of Americans get less than 7 hours of sleep a night.
Some try to catch up on sleep on weekends: a welcome but futile gesture for correcting the damage done from insufficient sleep during the week.
Sleep patterns owe to numerous factors, including culture and geoharmonic influences. Social zeitgebers, such as exposure to artificial light, are major determinants of sleep schedules for many people. A zeitgeber is an environmental cue that helps regulate circadian rhythm.
On average, people in Asia sleep less than those in Western nations. Asians tend to go to bed and wake up at later clock times than Westerners. This owes to social norms.
Sleep naturally follows the circadian clock, tempered by will. The body naturally wants to sleep in tune with proper zeitgebers. Resisting those cues can set up a biological cascade that disrupts the sleep cycle, with consequences for attentiveness and, in the longer term, health.
The body naturally tries to get the rest it needs. When mammals are deprived of sleep, parts of their brain spontaneously go into repose as best as they can. (Whether this occurs with other animals is not known.)
Sleep/awake timing differs among individuals by chronotype: the tendency to be a “morning lark” or “night owl.” Morning larks arise up early and are most alert early in the day. Night owls are most alert in the late evening and prefer to retire late. Most people are somewhere in between.
The hormone melatonin entrains the biological rhythms of alertness and rest. Melatonin is found in microbes, plants, and animals.
Sleep is an active state that is essential for the formation of lasting memories. ~ German psychologist and neurobiologist Susanne Diekelmann
Sleep cleans the brain. Sleep opens spigots that bathe the brain in fluids which sweep away potentially toxic buildup.
Sleep is essential to consolidate learning and memory. Implicit patterns become explicit knowledge through sleep, as glia process information gained during the day. This also applies in learning physical skills, such as playing the piano.
The subcortical regions are important in information consolidation, especially information linked to a motor memory trace. When consolidation level is measured after a period of sleep, the brain network of these areas functions with greater synchrony. Communication between the various regions of this network is better optimized. ~ Canadian neuropsychologist Karen Debas
Sleep is especially important for infant and child development. A child better transforms a day’s lessons into life experience via sleep than an adult.
When we are young, we have deep sleep that helps the brain store and retain new facts and information. But as we get older, the quality of our sleep deteriorates and prevents those memories from being saved by the brain at night. ~ American psychologist Matthew Walker
It is even possible to learn while asleep but doing so compromises memory consolidation. Memories are stabilized while snoozing via brain pathway replay. The hippocampus is especially active during memory replay.
Even a soul submerged in sleep is hard at work and helps make something of the world. ~ Greek philosopher Heraclitus
New memory-storing glia grow during sleep, with the greatest cell production during dreaming. Meanwhile, neural synapses are pruned.
Sleep on it. ~ proverb
Sleep facilitates problem-solving and decision-making. The more difficult the problem the more sleep helps. Part of this is allowing the unconscious to work things out, without the intrusion of bias that pervades the conscious mind.
A well-spent day brings happy sleep. ~ Leonardo da Vinci
Sleeping well is living well. Those with work to do sleep better than those out-of-work.
Artificial light can tinker with melatonin level. The light from computer screens can reduce melatonin levels by 22%. Melatonin suppression affects the sleep cycle and increases the risk of obesity and other disorders.
The worst thing in the world is to try to sleep and not to. ~ American author F. Scott Fitzgerald
The quality of life depends the quality of sleep. The ease of falling sleep reflects the quality of waking hours.
Sleep has social implications. The more socioeconomically deprived a neighborhood, the more erratic sleep patterns are. The disadvantaged suffer from too little or too much sleep.
Besides being tired, people who are sleep-deprived look less attractive and less healthy: and they are less healthy.
Sleep deprivation stresses the immune system. Sleep loss wears the body down, accelerating aging and memory loss. Loss of sleep, and especially dream time, presages Alzheimer’s disease.
Sleep deprivation for 24 hours can create a schizophrenia-like state in otherwise healthy adults. Older adults with sleep disorders are more likely to commit suicide. Reliance on sleep for cognitive functioning is why sleep deprivation is universally used in torture to break victims’ will.
Brain deterioration, bad memory and bad sleep are not independent, but instead significantly interrelated. ~ American neurobiologist Bryce Mander
Lack of sleep, stress, depression, or obesity cause sleepiness during the day. The body is demanding repose. Awareness is diminished, and thereby functionality of every sort.
A good laugh and a long sleep are the best cures in the doctor’s book. ~ Irish proverb