#126–Matthew Walker, Ph.D.: Sleep & immune function, chronotypes, hygiene tips, & his book

KEYPOINTS

• 01

  The pandemic changed sleep in three ways: how much, when, and dreaming

  Walker, a UC Berkeley neuroscientist, cites a sleep-tracking company's data on about 68,000 Americans claiming total sleep rose 20%. He is skeptical of that figure. More cautious peer-reviewed papers in Current Biology found a gain of about 15 minutes. On average, people went to bed about 30 minutes later and woke about 50 minutes later. He expects the data to split into two groups: people sleeping longer and better, and people sleeping less and worse because of job loss or anxiety.

• 02

  Lockdown shrank 'social jet lag'

  Social jet lag is the gap between weekday and weekend sleep timing. Walker says sleeping until 10 or 11 a.m. on weekends and then dragging the clock back two or three hours on Sunday night is 'like flying back and forth from San Francisco to New York every weekend.' Without commutes and school runs, sleep times became more uniform across the week. He calls it the 'revenge of the night owls,' who could finally sleep in line with their chronotype.

• 03

  There are two likely reasons people dreamed more

  First, sleeping in extends time in the late-night window where REM sleep is concentrated, so people get more REM cycles and vivid dreams. Second, Walker says REM sleep provides 'overnight therapy.' It isn't time that heals all wounds but 'time during REM sleep.' Because sleep responds to the day before, more emotionally difficult days can produce more REM. That may explain why Peter's dreams intensified even though his schedule didn't change.

• 04

  Alcohol can create an anxiety–REM spiral

  Walker says alcohol sedates rather than producing natural sleep, fragments sleep by stimulating fight-or-flight activity, and suppresses REM in the later hours. If wine numbs anxiety but removes the REM that processes emotions, the next day starts more anxious, which invites another glass. He stresses he isn't judging. Both men agree responses vary: fast metabolizers may clear an early drink before bedtime, while for others even a glass with a late lunch could crush their REM.

• 05

  Short sleep weakens immune defenses and vaccine response

  Walker cites a study finding that people sleeping under seven hours were almost three times as likely to be infected with a rhinovirus. In a prospective study of more than 70,000 women, those sleeping five hours or less were 70% more likely to develop pneumonia. Too little sleep in the week before a flu shot led to less than half the normal antibody response. Antibodies remained low 20–30 days later, so the response wasn't just delayed. He says whether this applies to COVID vaccines is unknown and should be tracked.

• 06

  Deep sleep depends on time awake; REM depends more on circadian timing

  Deep non-REM sleep, front-loaded in the first cycles, is driven mainly by adenosine sleep pressure. Higher cortisol typically means less deep sleep, and melatonin plays a smaller role. REM depends less on how long you've been awake and more on the circadian clock. Walker admits 'we have no idea why' sleep is organized with deep sleep early and REM late.

• 07

  Naps are a double-edged sword

  In Walker's studies, naps as short as 17 minutes improve learning and memory, and naps have shown cardiovascular benefits too. But naps release sleep pressure. His rule: if you struggle with sleep at night, avoid naps; if you sleep well, regular naps are fine. Some hunter-gatherer groups sleep six to six and a half hours at night plus a summer afternoon siesta. He calls this biphasic pattern different from the historical 'first and second sleep,' which he doesn't think reflects how our physiology is designed.

• 08

  Chronotype is largely genetic and shifts with age

  About 25–30% of people are morning types, 25–30% are evening types, and the rest are in between. Data from 23andMe and UK Biobank point to about nine contributing genes, mostly clock genes. Teenagers' rhythms shift later, so a 15-year-old who can't fall asleep at 10:30 isn't at fault. Older adults shift earlier, hence Florida's early-bird specials. A validated morningness–eveningness questionnaire works without genetic testing.

• 09

  Walker publicly corrected errors in Why We Sleep

  Examples he gives: a study described as 'over half a million' people actually analyzed 474,684. Another said to involve over 4,000 people analyzed 2,282 and measured 'cardiac events,' including angina requiring bypass, not 'cardiac arrests.' He also wrote that short sleep doubles cancer risk; he now says it is linked to doubling for specific cancers such as ovarian or lung, and calls the original phrasing 'poor judgment.' The figure of 30–40% of depressed patients responding to sleep deprivation was 45% in a later meta-analysis of prospective studies. Corrections went to a second edition, plus a blog post of more than 8,000 words.