This Podcast Will Kill You

Ep 198 Sleep Part 1: Sleeping with one eye open

January 20, 2026

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  • Human sleep is scientifically defined and categorized into two main phases, REM and non-REM, based on measurable electrical brain activity (EEG), muscle activity (EMG), and eye movement (EOG) recorded via a polysomnogram (PSG). 
  • Sleep drive, which increases throughout the day due to metabolic byproducts like adenosine, interacts with the circadian rhythm (governed by the SCN) to regulate when and how sleepy we feel. 
  • Sleep is an ancient and widespread physiological process across the animal kingdom, evidenced by its presence even in creatures without a central nervous system like jellyfish, suggesting it serves an absolute vital function. 
  • REM sleep is a relatively recent evolutionary development, primarily found in mammals and birds, and may be linked to cognitive activities or endothermia. 
  • Unihemispheric sleep, where one half of the brain sleeps while the other remains partially active (often with one eye open), is an adaptation seen in aquatic mammals (like whales, dolphins, seals) and some birds, often to maintain breathing, thermoregulation, or predator vigilance. 
  • Sleep duration across species is not random but is driven by evolutionary adaptations related to factors like predation risk, metabolism, immune function, gestation length, and brain mass relative to body size. 

Segments

Listener Sleep Paralysis Story
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(00:01:46)
  • Key Takeaway: Lucid dreaming training, specifically checking hands or clocks, can provide awareness to overcome sleep paralysis.
  • Summary: A listener shared a terrifying experience with sleep paralysis involving a looming figure, which he eventually overcame by actively grabbing the figure’s wrist while attempting lucid dreaming techniques. He learned that lucid dreaming cues, like checking one’s hands or clocks, can confirm one is dreaming. This specific confrontation ended his recurring sleep paralysis episodes for several years.
Episode Outline and Beverage
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(00:06:01)
  • Key Takeaway: This first part of ‘Ep 198 Sleep Part 1: Sleeping with one eye open’ focuses on defining sleep, brain activity, and animal sleep patterns.
  • Summary: The hosts introduced the two-part series on sleep, confirming this episode covers the definition of sleep, brain activity during sleep, and animal sleep. The second episode will cover sleep deprivation consequences and the history of human sleep. They introduced the non-alcoholic ‘Pillow Talk’ Placebarita, a chamomile tea with ginger honey syrup, as the accompanying beverage.
Defining Sleep Stages
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(00:13:07)
  • Key Takeaway: Sleep is medically defined by EEG patterns, transitioning from fast, low-amplitude awake waves (beta/alpha) to slower, higher-amplitude waves (delta) in deep sleep.
  • Summary: Sleep is defined by reduced voluntary motor activity and decreased response to stimulation, but medically by EEG patterns measured alongside muscle (EMG) and eye movement (EOG) via a polysomnogram (PSG). Awake states show fast, low-amplitude waves (beta/alpha), which slow down into theta waves during Stage 1 and 2 non-REM sleep. Deep sleep (Stage 3) is characterized by high-amplitude, low-frequency delta waves.
REM Sleep Characteristics
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(00:21:47)
  • Key Takeaway: REM sleep EEG patterns resemble being awake (beta waves), but the body experiences atonia (muscle paralysis) and rapid eye movements.
  • Summary: In contrast to non-REM sleep, REM sleep shows an EEG pattern similar to wakefulness, with elevated blood pressure and heart rate. The defining features of REM are rapid horizontal eye movements and atonic skeletal muscle activity, which prevents movement. Sleepwalking occurs during deep sleep, whereas sleep paralysis is linked to waking up while the body is still in REM atonia.
Sleep Cycles and Proportions
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(00:23:53)
  • Key Takeaway: A typical adult sleep cycle lasts 90-100 minutes, cycling through NREM stages before entering REM, with deep sleep dominating early in the night and REM periods lengthening later.
  • Summary: A complete sleep cycle moves from Stage 1 to 2, then 3 (deep sleep), back through 2, and finally into REM sleep. Adults typically get 20-30% REM sleep, while deep sleep constitutes about 15-20% of total sleep. Sleep-deprived individuals prioritize deep sleep recovery on subsequent nights.
Wearable Sleep Tracker Accuracy
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(00:27:07)
  • Key Takeaway: Consumer sleep tracking devices are generally poor at distinguishing between specific sleep stages on an individual night, despite decent overall duration accuracy.
  • Summary: Independent research suggests that while wearable devices correlate strongly with clinical PSGs on an average population level, they exhibit substantial individual-level inaccuracies night-to-night. This variability prohibits their use in clinical sleep medicine, especially for diagnosing disorders like sleep apnea, though they are decent at measuring total awake versus sleep duration.
Drivers of Sleep and Wakefulness
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(00:30:07)
  • Key Takeaway: Wakefulness is promoted by neurotransmitters released from the brainstem (like histamine and dopamine), while sleepiness builds up via metabolic byproducts like adenosine.
  • Summary: The brainstem releases wakefulness-promoting neurotransmitters, which are inhibited by signals from the frontal brain as the day progresses. Adenosine, a byproduct of metabolism, builds up and increases sleep pressure by binding to inhibitory receptors. Caffeine functions by blocking these adenosine receptors, tricking the brain into feeling less sleepy.
Sleep Variation Across Lifespan
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(00:35:12)
  • Key Takeaway: Infants lack a defined circadian clock, exhibiting random, short sleep cycles (shorter than 90 minutes) until 6 to 12 months of age when 24-hour patterns consolidate.
  • Summary: Sleep needs change drastically across the lifespan; teenagers experience a natural shift toward night owl tendencies due to circadian rhythm changes. Infants sleep up to 16 hours daily in random patterns because their circadian clock is not yet established. It takes the first six to twelve months for infants to consolidate sleep into a recognizable 24-hour pattern.
Hypotheses for Sleep Function
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(00:39:00)
  • Key Takeaway: The precise function of sleep remains unknown, but leading hypotheses include energy conservation, clearing metabolic buildup (brain detox via CSF flow), and consolidation of memory.
  • Summary: Sleep is not primarily for energy conservation, as metabolism during REM sleep is similar to wakefulness. One major theory suggests sleep facilitates a ‘brain detox’ by increasing cerebral spinal fluid flow to clear accumulated neurotransmitters. Sleep is also strongly implicated in sleep-dependent memory processing, though the exact necessity of sleep for this function remains debated.
Animal Sleep Diversity
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(00:43:17)
  • Key Takeaway: Sleep is ubiquitous across the animal kingdom, with criteria like prolonged quiescence and rebound after deprivation defining it, and some species like jellyfish exhibiting sleep before central nervous systems evolved.
  • Summary: Researchers use five behavioral criteria—including species-specific posture and increased arousal threshold—to confirm sleep in animals, leading to the conclusion that sleep is ancient, likely evolving before central nervous systems. Sleep duration varies widely, from four hours in elephants to 22 hours in koalas, and some species exhibit unihemispheric sleep, allowing them to remain partially alert.
REM Sleep and Animal Distribution
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(00:56:29)
  • Key Takeaway: REM sleep evolved independently in mammals and birds, possibly due to its importance for endothermia.
  • Summary: Mammals and birds are the only animals known to have REM sleep, with the exception of the cuttlefish. REM sleep is thought to have evolved after non-REM sleep in these groups independently. This suggests REM sleep might be crucial for cognitive activities or maintaining warm-bloodedness (endothermia).
Sleep Cycle Duration Variation
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(00:57:10)
  • Key Takeaway: Sleep cycle lengths between REM and non-REM stages vary drastically across species, such as the human 90-minute cycle versus the chinchilla’s six-minute cycle.
  • Summary: Sleep cycles, the time taken to move between REM and non-REM stages, show extreme variation in the animal kingdom. Humans average about 90 minutes per cycle. In contrast, the chinchilla completes an entire sleep cycle in just six minutes.
Unihemispheric Sleep Mechanics
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(00:57:29)
  • Key Takeaway: Unihemispheric sleep allows aquatic mammals and certain birds to remain partially alert while resting, often correlating with a lack of recognizable REM sleep.
  • Summary: Unihemispheric sleep is the ability to sleep with only half the brain active, observed in aquatic mammals like whales, dolphins, and seals, as well as some migrating birds. These aquatic mammals often do not exhibit REM sleep, or it is not in a recognizable form. Reasons for this include the need to breathe, predator lookout, reducing heat loss, and staying close to group members.
Fur Seal Sleep Adaptations
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(01:00:47)
  • Key Takeaway: Fur seals exhibit different sleep patterns depending on their environment, using unihemispheric sleep in water but full REM/non-REM cycling on land.
  • Summary: When in water, fur seals float on their sides, paddle constantly with one flipper, and engage in unihemispheric sleep with one eye open. When on land, they sleep like terrestrial mammals, closing both eyes and cycling through REM and non-REM stages.
Newborn Dolphin Sleep Deprivation
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(00:59:23)
  • Key Takeaway: Newborn dolphins and orcas are continuously active for about four weeks immediately after birth, showing no sleep.
  • Summary: Unlike human babies, newborn dolphins and orcas are continuously active for approximately four weeks without any sleep, alongside their mothers. This intense activity period likely occurs because this time is extremely vulnerable for the babies. Sleep gradually returns to these newborns only after this initial period.
Drivers of Sleep Duration
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(01:03:13)
  • Key Takeaway: Key drivers determining how long a species sleeps include predation risk, metabolism, immune function, gestation length, and brain mass.
  • Summary: When controlling for relatedness, several strong drivers emerge that influence sleep patterns across species. Prey species or those in vulnerable locations tend to have shorter sleep durations due to predator vigilance. Animals with faster metabolisms relative to body size also sleep less, and smaller animals are often polyphasic sleepers.
Predation Risk and Sleep
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(01:04:29)
  • Key Takeaway: Vulnerability to predators directly correlates with shorter sleep durations, suggesting light or reduced sleep is an adaptive response to danger.
  • Summary: Sleep inherently makes an animal vulnerable because senses are dulled and movement ceases. Prey species or those sleeping in exposed areas generally exhibit shorter sleep durations to lower this risk. Some researchers suggest that light sleepers or those with insomnia may have vigilance mechanisms related to predator avoidance operating on overdrive.
Metabolism and Sleep Patterns
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(01:06:41)
  • Key Takeaway: Herbivores, often prey species with faster metabolisms, spend significant time foraging, leading to shorter overall sleep compared to predators.
  • Summary: Herbivores often have faster metabolisms and less calorically dense food, requiring more frequent foraging time across the landscape. Animals with faster metabolisms relative to their body size tend to sleep less overall. Smaller animals, which often have faster relative metabolisms, tend to be polyphasic sleepers, sleeping in short bursts throughout the day.
Primate Sleep Extremes
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(01:09:12)
  • Key Takeaway: Humans have one of the shortest sleep durations among all primate species, contrasting with species like the night monkey which sleeps up to 17 hours.
  • Summary: Humans share a similar sleep duration (7-8 hours) and REM proportion (about 2 hours) with the Eastern American mole. Among primates, humans are an exception, having some of the shortest sleep durations recorded. Night monkeys, conversely, sleep about 17 hours but maintain the same amount of REM sleep as humans.
Human Sleep Model Prediction
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(01:10:28)
  • Key Takeaway: Based on models incorporating primate characteristics like brain size and predation risk, humans are predicted to sleep about nine and a half hours nightly, suggesting current average human sleep is highly compressed.
  • Summary: A model based on primate characteristics (body size, brain size, diet, etc.) predicts that humans should sleep around nine and a half hours per night. The average human sleep duration is typically six to seven hours, suggesting humans may be more efficient or intense sleepers than predicted by primate comparisons. Great apes generally sleep more than humans but less than night monkeys.
Great Ape Nest Building
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(01:11:23)
  • Key Takeaway: All great apes build individual nests nightly, a behavior potentially linked to enhanced cognition, predator avoidance, and thermoregulation.
  • Summary: All great apes build individual nests, usually in trees, which suggests this behavior emerged 14 to 18 million years ago. Nest placement is influenced by predator presence, with chimpanzees building higher nests when predators are near. Nests also aid in thermoregulation by allowing adjustment of insulation and may help deter biting insects.
Ground Sleeping and Human Evolution
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(01:14:01)
  • Key Takeaway: Sleeping on the ground, despite increased vulnerability, may have allowed humans deeper sleep, enabling shorter sleep durations and facilitating the evolution of language via increased social time.
  • Summary: Sleeping on the ground, unlike in trees, provided a stable surface that might have allowed for deeper sleep and reduced vigilance needs, potentially shortening necessary sleep duration. The control of fire further reduced risks from predators and insects, allowing for longer periods of nighttime socialization. This extended social time may have promoted the evolution of language.