Curiosity Weekly: "This is Your Brain on Sleep"

Host: Dr. Samantha Yammine
Guest: Dr. Jeffrey Iliff, Professor of Psychiatry, Behavioral Sciences & Neurology, University of Washington
Date: November 26, 2025

Episode Overview

This episode explores the fascinating, multifaceted science of sleep: why we need it, how it shapes our brains and bodies, and the remarkable processes at play during the nightly cycle. Dr. Samantha Yammine interviews Dr. Jeffrey Iliff, co-discoverer of the brain's glymphatic system, to delve into the “brain-cleaning” function of sleep, the molecular drivers of sleep across species, connections to brain health and disorders like Alzheimer’s, and insights from cutting-edge research on sleep diversity. The show also unpacks a new study proposing five different sleep types, their brain patterns, and their connections to mental health.

Key Discussion Points & Insights

1. The Science and Evolution of Sleep ([02:10]–[06:37])

• Sleep is universal among humans, but far from fully understood—research has long debated its evolutionary functions: memory consolidation, metabolism, immune function, brain “pruning,” and detoxification.
• Animals show extraordinary diversity in sleep:
  • Jellyfish don’t sleep in the human sense, but display rhythms akin to rest ([03:22]).
  • Dolphins sleep one brain hemisphere at a time—“Talk about multitasking!” (Dr. Yammine, [02:45]).
  • Placozoans (simplistic, brainless animals) slow down at night, suggesting even the simplest creatures have circadian cycles ([04:24]).
• Groundbreaking research links sleep drive to mitochondria—cellular powerhouses—via electron “leakage”:
  • “When [researchers] reduced the electron leakage, the flies slept less, and when they increased it, they slept more.” (Dr. Yammine, [05:37])
  • Implies sleep’s deepest roots may be metabolic, with other benefits piggybacking.

2. What’s Happening in Your Brain during Sleep? ([09:31])

• Our brains shift through “three or four really different identifiable electrophysiological states” during sleep (Dr. Iliff, [10:27]).
• REM (dream) sleep is best known, but “slow wave sleep” is the deepest phase, marked by synchronized neuronal activity/inactivity oscillations ([10:56]).
• “It seems like [the] brain is doing very, very different things through the course of the night…biologically, what’s happening…we’re only just starting to get a sense of.” (Dr. Iliff, [11:18])

3. The Glymphatic System—Your Brain's Nightly Cleanse ([11:32]–[13:28])

• Dr. Iliff’s research identified the glymphatic system: a fluid-based process by which the brain removes metabolic waste.
  • During sleep—specifically, slow wave sleep—your brain enters “cleaning mode,” flushing out waste accumulated during the day ([12:29]).
  • “[Sleep] allows the clearance of wastes that accumulate in the brain through the course of the waking day.” (Dr. Iliff, [12:53])
• Glymphatic vs. Lymphatic systems:
  • The lymphatic system (body-wide) clears waste and supports immunity, but the brain has no lymphatic vessels. Instead, glial cells support the “glymphatic” cleaning process within the brain. (Dr. Iliff, [13:28])
  • Later research showed lymphatic vessels do exist around (but not in) the brain and assist in fluid clearance ([14:10]).

4. Waste Clearance, Sleep “Quality,” and Brain Function ([14:46])

• Lack of sleep impairs brain “pruning” (synaptic downscaling) and may limit glymphatic waste removal, potentially causing post-bad-night grogginess and cognitive dysfunction.
  • “How much…actually contributes to how you feel after you don’t sleep and how you’re thinking after you don’t sleep? We actually don’t know the answer to that yet.” (Dr. Iliff, [15:52])
• Possible link between migraines and glymphatic dysfunction has been found in animals, not yet in humans ([16:39]).

5. How Glymphatic Research Is Conducted ([16:57])

• In animals: Tracers (fluorescent or MRI) are injected, then tracked through brain tissue.
• In humans: Usually MRI-based, since invasive tracers aren’t practical. Researchers track water movement in the brain and measure amyloid/tau protein clearance via blood samples ([17:10]).
• Fluctuations in blood levels of these proteins may indicate the brain’s overnight “cleaning” cycles ([19:33]).

6. Sleep, Dementia, and Alzheimer’s ([20:02])

• Poor sleep is a “clear risk factor” for dementia—including Alzheimer’s—based on large-scale epidemiology ([20:21]). Notably:
  • Both too little (<6h) and too much (>9h) sleep elevate dementia risk (“U-shaped” curve).
  • Quality matters as much as quantity—fragmented, “crappy” sleep increases risk ([21:08]).
  • Midlife sleep (age 40s-50s) is most predictive for later dementia risk ([21:34]).
  • “It goes both ways”—as people develop dementia, their sleep also worsens.

7. How Do You Measure “Good” Sleep? ([22:01])

• Despite new data (from wearables, etc.), we still don’t precisely know what constitutes “good” or “sufficient” sleep—subjective satisfaction often doesn’t align with objective sleep stage data.
  • Aging reduces slow wave/deep sleep, but many elderly people don’t feel impaired ([22:11]–[23:57]).
  • 30 years ago, the only available measure was “Do you sleep well?”—now we have copious quantitative data, but it’ll be decades before we know how it correlates to health outcomes ([23:57]).

8. Are Sleep Trackers Useful? ([24:22])

• Sleep tracking wearables (Garmin, Apple Watch, etc.) are around 70–85% accurate compared to gold standard studies. Useful for individual feedback, but not perfect ([24:57]).
• “[Within] an individual, I think it can be very useful for understanding what are the factors that affect your sleep...when you sample it every day you...get a sense of cause and effect.” (Dr. Iliff, [26:39])

9. Do Some People Truly Need Less Sleep? ([27:13])

• There is both behavioral and genetic variability in sleep need—some people function on minimal sleep, but may not realize they could be even more functional with more rest ([27:20]).
  • “There does seem to be a genetic determinant of how much sleep you need and how quickly you can dissipate sleep need.” (Dr. Iliff, [27:57])
• It’s unknown whether “short sleepers” are protected from dementia risk ([28:17]).

10. Weekend “Catch Up” Sleep and Social Jet Lag ([28:41])

• Your body prefers regularity; erratic schedules or “recovery sleep” on weekends causes “social jet lag”—akin to a weekly transatlantic flight.
  • “Essentially it’s like you’re taking a transatlantic flight every weekend…you’re just kind of always jet lagged.” (Dr. Iliff, [29:53])

11. Practical Advice & Misconceptions ([31:01])

• Sleep hygiene advice is widely published (“Don’t drink caffeine after 3pm…quiet, cool room…don’t work in your bedroom…”).
• Two lesser-discussed tips:
  1. Try eight hours for two weeks as an experiment. Dr. Iliff did, and “it was crazy…like I was a totally different human than I thought I could be.” ([32:13])
  2. Think about the long view: “Your sleep habits in your 40s and 50s...is going to determine what your brain is like when you’re in your 60s and 70s and 80s.” ([33:06])

Notable Quotes & Memorable Moments

• "Most animals sleep, but not all of them. And we’re not exactly sure why."
  — Dr. Samantha Yammine ([02:34])

• "The brain is actually shifting into a kind of cleaning mode where that flushing of fluid...is actually turning on during sleep."
  — Dr. Jeffrey Iliff ([12:34])

• "You could imagine getting eight hours of sleep, but maybe it’s of a different quality than another person who gets eight hours."
  — Dr. Jeffrey Iliff ([23:29])

• "[Wearables] are right within me, right? So if I have a terrible night of sleep...my watch knows…my watch tells me."
  — Dr. Jeffrey Iliff ([25:39])

• “Try the experiment…set a going to bed alarm and a waking up alarm so that you would get eight hours of sleep…just do it for two weeks and see if that doesn’t unlock more function than you thought you had.”
  — Dr. Jeffrey Iliff ([32:12])

• “Your sleep habits in your 40s and 50s…is going to determine what your brain is like when you’re in your 60s and 70s and 80s.”
  — Dr. Jeffrey Iliff ([33:06])

Highlighted Segment Timestamps

• [02:10] – Introduction to animal sleep variation and the evolutionary context
• [09:31] – Why sleep matters biologically and cognitively
• [10:27] – Brain’s electrophysiological stages during sleep (REM, slow wave, etc.)
• [11:45] – The glymphatic system: discovery and function
• [13:28] – The confusing terminology: glymphatic vs. lymphatic systems
• [15:58] – Can poor glymphatic function explain morning headaches?
• [17:10] – How researchers study glymphatic activity in animals & humans
• [20:21] – Sleep as a dementia and Alzheimer’s risk factor
• [22:11] – What actually defines “good” or “enough” sleep?
• [24:57] – The practical usefulness of sleep-tracking tech
• [27:20] – Do we really need as much sleep as we think?
• [28:54] – The pitfalls of sleeping in on weekends (“social jet lag”)
• [31:16]–[33:27] – Dr. Iliff’s practical takeaways and personal experience

New Research: The Five Types of Sleep ([36:40])

• Concordia University study identified five sleep patterns among young adults:
  1. Low satisfaction/psychological distress — Trouble falling/staying asleep, lower cognitive performance, mental health symptoms.
  2. Sleep resilient — Okay sleep despite psychological symptoms.
  3. Frequent sleep aid users — Few sleep complaints but lower visual memory.
  4. Short sleepers (<6–7h) — Lower accuracy, increased aggression, higher brain network connectivity.
  5. Fragmented sleepers — High levels of anxiety, substance misuse, worse cognitive scores.
• “There are a lot of ways that sleep affects our bodies, our brains, and our mental health, and…we need to be looking at it from all angles.” (Dr. Yammine, [39:53])

Conclusion

This episode demystifies sleep as a complex, essential biological process, not just a passive shutdown. The conversation ranges from ancient cellular functions to modern biometrics, the “garbage collection” role of the glymphatic system, implications for brain health and disease, and the marvelously diverse animal world. Dr. Iliff’s firsthand advice is clear: try prioritizing eight hours for yourself, observe your own patterns with curiosity, and remember that what you do tonight may shape your brain decades from now.