Six Hours of Sleep: What a 25-Year Study Found

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They asked the same question at three ages

The researchers studied sleep duration at ages 50, 60 and 70. The wider analysis followed 7,959 adults and recorded 521 cases of dementia. Repeated reports let the team compare people who consistently reported six hours or less with people who consistently reported seven hours.

What the 30% measures

The adjusted hazard ratio was 1.30, with a 95% confidence interval of 1.00 to 1.69. This compares the rate of newly recorded dementia cases between groups during follow-up. It does not mean 30 out of 100 people developed dementia. The outcome included dementia from any cause.

The timing matters

Sleep can change before a dementia diagnosis. The researchers used reports from midlife to put more time between the sleep measurement and the outcome. The study still leaves open how much of the association could reflect other differences between the groups.

Sleep timing and cortisol

In a small laboratory crossover study, 13 healthy adults followed normal, earlier, and later sleep schedules under controlled conditions. During the delayed schedule, participants with less REM sleep in the second half of the night tended to have a larger cortisol response after lunch. Researchers did not find the same relationship under the normal or advanced schedule. The study examined sleep timing and hormone responses. Whitehall tracked sleep duration and later dementia diagnoses.

Inside the sleeping brain

Fultz and colleagues used simultaneous EEG and MRI to record slow waves of cerebrospinal fluid during non-REM sleep. The experiment measured fluid movement. It did not track amyloid clearance or later dementia.

What a genetic result measures

Chris Hemsworth disclosed two copies of APOE4 after testing on Limitless. Fortea and colleagues reported that around 75% of people with two copies in the research cohorts had positive amyloid scans by age 65. That is a biomarker result. It is distinct from the probability of symptoms by that age.

References

• Gonnissen et al., PLOS ONE, 2013. Sleep architecture, circadian timing, and cortisol
• Fultz et al., Science, 2019. Coupled electrical, blood flow and fluid oscillations during sleep
• Fortea et al., Nature Medicine, 2024. APOE4 homozygosity and Alzheimer’s biomarkers
• Chris Hemsworth’s account of his genetic test
• Sabia et al., Nature Communications, 2021. Sleep duration and incident dementia
• HealthBrew guide to sleep regularity