#47–Matt Walker, Ph.D. on Sleep (Part 1 of 3): Dangers of poor sleep, dementia risk, mental health..

KEYPOINTS

• 01

  Walker found his field by recording dementia patients asleep, not awake

  As a PhD student in London, Walker spent two years failing to tell dementia types apart from waking brainwaves. Reading journals one Sunday, he noticed that some dementias attack the brain's sleep-generating centres while others spare them until late in the disease. He set up a sleep lab, got results, and began asking whether disrupted sleep is a symptom, a predictor or a cause of Alzheimer's. He now believes the evidence points to a causal role.

• 02

  Deep sleep powers the brain's 'sewage system'

  The glymphatic system is a cleaning network run by glial cells. In deep sleep, Walker says, those cells shrink dramatically, giving cerebrospinal fluid room to flush out the day's metabolic waste. He compares it to Manhattan's buildings shrinking so a flood can wash the streets. That waste includes beta-amyloid, one of the two core proteins in Alzheimer's, so skimping on sleep lets it build up 'like compound interest on a loan'.

• 03

  The evidence chain runs from brain scans to spinal taps

  In people in their 70s and 80s, PET scans show visibly more amyloid in those sleeping seven hours or less than in those sleeping more. That is associational, he notes. Rats deprived specifically of deep sleep, or with fragmented sleep, show immediate amyloid build-up. In humans, cerebrospinal fluid taken after one night of selectively disrupted deep sleep shows significant rises in amyloid and tau. That last finding is what led Walker to say publicly that sleep is a critical part of preventing Alzheimer's.

• 04

  Sleep has four pillars

  Walker proposes regularity (avoiding 'social jet lag' from shifting bedtimes), continuity (unfragmented sleep), quantity (total hours and hours in each stage) and quality, meaning the electrical signature of sleep. He stresses that eight hours in bed doesn't guarantee eight hours of good-quality deep sleep, especially with caffeine or alcohol. He expects different sleep features to predict different subtypes of Alzheimer's.

• 05

  Non-REM and REM battle in 90-minute cycles

  Non-REM sleep has four stages; stages 3 and 4 are the deep, restorative ones. REM is dream sleep, with rapid side-to-side eye movements. Awake, brainwaves are fast, at about 50–60 cycles per second, and small, like a stadium crowd chattering before the game. With eyes closed, the visual cortex at the back of the brain slows to an alpha rhythm of about 10 cycles per second. The two types of sleep alternate in cycles of about 90 minutes all night.

• 06

  Deep sleep dominates early in the night, REM late

  Walker likens the night to a buffet. From about 9 p.m. to 3 a.m. the brain preferentially takes deep non-REM sleep; after about 3 a.m., REM and stage 2. So sleeping 9 p.m. to 3 a.m. preserves deep sleep but cuts most REM, while very late sleepers get the reverse. Attia sees this on his ring: more deep sleep after early nights back in California, more REM in New York.

• 07

  Memory needs sleep both after and before learning

  Deep non-REM sleep after learning 'hits the save button' on new memories, so getting the first half of the night protects them. Stage 2 sleep, rich in brief bursts of activity called sleep spindles, peaks in the last two hours of the night and prepares the brain to take in new memories the next day. Cutting the morning short reduces that capacity.

• 08

  Lack of REM sleep hits mental health hardest

  Walker calls sleep 'emotional first aid'. He links REM loss most strongly to depression, anxiety and suicidality. In young teenagers, he says, insufficient sleep is one of the strongest predictors of suicidal thoughts, attempts and deaths. Sleep problems and anxiety feed each other, but a four-year-old's meltdown after one bad night suggests sleep can drive the problem.

• 09

  Drowsy driving can be deadlier than drunk driving

  Microsleeps, brief lapses when the eyelids partly close, become common on six or seven hours of sleep. Walker says drowsy driving causes more crashes than drugs and alcohol combined. Drunk drivers usually react, but too late. During a microsleep there's no braking or steering at all, just 'a two-ton missile' at 65 mph.