This Podcast Will Kill You

Ep 199 Sleep Part 2: Predictably unpredictable

January 27, 2026

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  • Historical evidence suggests that pre-industrial humans commonly practiced segmented (biphasic) sleep, waking for an hour or two around 2 a.m. for activities like socializing or chores, a pattern that has been largely lost due to industrialization. 
  • Modern society fosters 'competitive sleeplessness' and the medicalization of sleep, leading to anxiety and the over-pathologizing of natural human sleep variation, which is inherently flexible. 
  • While short sleep duration (less than seven hours) is clearly linked to negative health outcomes like accidents and cardiovascular disease, the ideal total sleep duration can be achieved through segmented sleep or naps, not just one continuous block. 
  • Current scientific data on sleep quality is limited, relying heavily on total sleep duration, making it difficult to distinguish the health impacts of sleep loss due to external circumstances versus internal issues like insomnia. 
  • National Sleep Foundation guidelines for 'quality sleep' (e.g., falling asleep in 15-20 minutes, napping being an indicator of poor sleep) are based on averages and exhibit significant expert disagreement on specific metrics. 
  • Sleep deprivation impairs an individual's ability to recognize their own deficits, similar to alcohol intoxication, meaning those who are sleep deprived often do not realize the extent of their impairment. 

Segments

Listener Story: Drowsy Driving
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(00:01:46)
  • Key Takeaway: Severe sleep deprivation led a recent college graduate to cause a minor car accident, totaling her car.
  • Summary: A listener named Kelly recounted staying awake through finals week using caffeine pills, leading to drowsy driving afterward. While the accident only damaged her car, Betty, the experience reinforced her caution against driving fatigued. This personal anecdote highlights the immediate, tangible risks of severe sleep deprivation.
Episode Introduction and Recap
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(00:04:10)
  • Key Takeaway: This second part of the sleep series focuses on historical human sleep patterns and the consequences of sleep disruption.
  • Summary: The hosts, Erin Welsh and Erin Alman Updike, recap that the previous episode covered the stages and functions of sleep. This episode will explore how humans slept historically and detail the consequences of insufficient or excessive sleep. They also introduce the ‘Pillow Talk’ chamomile tea cocktail.
Historical Sleep Evidence
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(00:13:52)
  • Key Takeaway: Evidence for pre-industrial segmented sleep comes from historical writings referencing ‘first sleep’ and ‘second sleep’ separated by a period of wakefulness called ‘watch’ or ‘watching’.
  • Summary: Since sleep leaves no fossil record, researchers examine modern hunter-gatherer groups and historical texts to understand past sleep. Robert Louis Stevenson observed a common 2 a.m. awakening among rural populations, which aligns with historical references to biphasic sleep patterns. The first sleep was sometimes called ‘dead sleep,’ while the intervening wakeful period was ‘watch.’
Pre-Industrial Evening Activities
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(00:20:35)
  • Key Takeaway: Pre-industrial evenings were often boisterous social times, and people valued sleep as a peaceful respite, sometimes engaging in activities like starting bread dough during the middle-of-the-night break.
  • Summary: Contrary to the idea of immediately sleeping at sunset, pre-industrial life included socializing by hearth light or at neighbors’ houses. Historical sources show people valued sleep, with one Danish pastor’s inscription setting a strict 11 p.m. cutoff for guests. Bedding rituals included warming beds with coals and removing pests like lice or bed bugs.
Romanticizing Past Sleep Quality
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(00:24:27)
  • Key Takeaway: Despite elaborate bedtime rituals, historical accounts frequently describe sleep as restless or troubled due to poor health, shared sleeping spaces, and lack of modern sanitation.
  • Summary: The assumption that ancestors slept better than modern people is likely false, as many historical descriptions used words like ‘restless.’ Poorer classes often shared beds with family and animals, and widespread illness made restful sleep difficult. Daytime napping was common, suggesting that continuous, high-quality sleep was not the norm.
Industrialization and Sleep Shift
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(00:27:01)
  • Key Takeaway: The Industrial Revolution, driven by artificial light and the need for worker productivity, caused the widespread abandonment of segmented sleep in favor of the monolithic ‘8-8-8’ rule.
  • Summary: Artificial light, shift work, and urban nightlife caused segmented sleep to fade by the 20th century, replaced by the ideal of eight hours for work, eight for sleep, and eight for self. This shift coincided with the medicalization of sleep, turning it into an object to be managed or optimized for industrial efficiency.
Sleep Flexibility vs. Inflexible Society
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(00:32:01)
  • Key Takeaway: Humans are ‘predictably unpredictable sleepers’ whose evolutionary history favored flexible sleep timing for social and protective benefits, a flexibility now undermined by rigid societal structures.
  • Summary: Nighttime conversations historically focused on abstract and creative topics, benefiting species development. While flexibility in sleep timing (like chronotypes or napping) is evolutionarily beneficial, modern society demands rigid adherence to schedules (e.g., (7:20) a.m. school start), making individual flexibility a source of failure.
Modern Sleep Obsession and Medicalization
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(00:36:17)
  • Key Takeaway: The medicalization of sleep has created a narrow definition of ’normal’ sleep, leading to the over-treatment of normal variation and turning the restorative process of sleep into a source of stress.
  • Summary: Thomas Edison exemplified the toxic Protestant work ethic by viewing sleep loss as a loss of ‘vitality and opportunities,’ a sentiment that persists today. While medical advances have helped diagnose true disorders like sleep apnea, the focus on optimization creates a cycle where worrying about sleep worsens its quality.
Sleep Duration Data and Caveats
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(00:46:09)
  • Key Takeaway: U.S. data suggests average sleep duration has declined since the 1980s, but the negative health consequences associated with short sleep (under 7 hours) are often based on total duration, not necessarily continuous duration.
  • Summary: Studies show that consistently sleeping less than seven hours or more than nine hours is associated with increased long-term health risks, though the causality of long sleep is often questioned (reverse causation). Crucially, data confirms that split sleep schedules and naps count toward the total required sleep duration for performance improvement.
Data Limitations and Sleep Quality
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(00:55:24)
  • Key Takeaway: Data primarily focuses on negative outcomes of insufficient sleep duration, leaving gaps in understanding what constitutes ‘good sleep’.
  • Summary: Existing data on sleep’s negative effects largely relies on total sleep duration and deprivation, not on defining what makes sleep ‘good.’ A key question raised is whether six hours of sleep due to external life circumstances has the same health outcome as six hours when eight were desired but unattainable. Currently, the literature lacks the necessary disentanglement to compare these different causes of short sleep duration.
Defining Quality Sleep Metrics
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(00:57:13)
  • Key Takeaway: National Sleep Foundation guidelines define quality sleep by metrics like falling asleep within 15-20 minutes and specific REM/deep sleep percentages.
  • Summary: Quality sleep, according to the National Sleep Foundation guidelines (circa 2017), involves falling asleep relatively quickly (15-20 minutes); falling asleep too fast (under 8 minutes) may signal sleep deprivation. Good quality sleep should involve waking up no more than once and being awake for less than 20 minutes total after initially falling asleep. Consensus averages suggest REM sleep should be 20-30% and deep sleep 15-20% of total duration, though experts disagree on the exact ranges.
Napping and Biphasic Sleep Bias
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(00:59:49)
  • Key Takeaway: Guidelines suggest daytime napping (especially over 90-100 minutes) and split sleep schedules indicate poor sleep quality, reflecting a modern, industrial bias.
  • Summary: Guidelines indicate that napping during the day (if not an infant) is an indicator of poor sleep quality, particularly naps exceeding one full sleep cycle (90-100 minutes). This implies that biphasic or split sleep schedules would be classified as poor quality sleep under these modern metrics. This emphasis feels rooted in the industrial revolution’s demands rather than historical human sleep patterns.
Sleep Disorders Overview
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(01:04:13)
  • Key Takeaway: Adults generally need 7-8 hours of sleep, and major sleep disorders include insomnia, breathing disorders like sleep apnea, and central hypersomnias like narcolepsy.
  • Summary: While historical pre-industrial societies averaged 5-7 hours, modern medicine often considers less than 7 hours low, though averages are messy. Insomnia (difficulty falling or staying asleep) affects 10% chronically in the U.S., while obstructive sleep apnea, which fragments sleep without necessarily changing duration, affects nearly 15% and has severe cardiovascular consequences. Narcolepsy involves daytime sleep attacks and fragmented nighttime sleep, sometimes accompanied by cataplexy.
Self-Recognition of Sleep Deficit
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(01:08:27)
  • Key Takeaway: Acute sleep deprivation reduces an individual’s ability to accurately assess their own level of impairment, similar to being drunk.
  • Summary: There is often a disconnect between objective sleep measurements (like from a wearable device) and how rested a person subjectively feels. Acutely sleep-deprived individuals often believe they are functioning normally, even when their decision-making is impaired, mirroring the effect of alcohol intoxication. This lack of self-recognition complicates personal assessment of sleep needs.
Societal Pressures and Ideal Sleep
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(01:13:58)
  • Key Takeaway: The emphasis on achieving a standardized ‘ideal’ sleep pattern creates pressure and ignores non-pathogenic variations in human sleep needs.
  • Summary: Numerous factors like diet, caffeine, alcohol, stress, and environment work against achieving optimal sleep. The societal emphasis on achieving the ‘quote-unquote right’ sleep creates pressure and disconnects individuals from assessing their own feelings of restfulness. Public health guidance must generalize, but this generalization struggles to account for individual, non-pathogenic variations in sleep requirements.