Third piece of body-homework, and the one that changed my behavior fastest.
Sleep isn't downtime. It's the maintenance shift, the brain flushes metabolic waste (in mice, the spaces between brain cells expand about 60% during sleep and waste clearance roughly doubles), memories consolidate, growth hormone pulses for tissue repair.
But late nights add a second problem on top of short sleep: misaligned clocks. Light at night suppresses melatonin, one study found ordinary room light before bed cut melatonin duration by about 90 minutes. And eating late tells your liver and gut it's daytime while your brain disagrees. The clocks desync.
The measurable effects of short sleep stacked up fast when I read them. Hunger hormones tilt toward eating more, the "I'm full" signal down 18%, the "I'm hungry" signal up 28%, which makes any calorie target materially harder. Thinking dulls to roughly the level of mild intoxication: about 17 hours awake performs like a blood-alcohol level of 0.05%. And over years, short sleep roughly doubles the odds of developing hypertension in midlife.
The through-line across all three of these inquiries, alcohol, smoking, sleep, is the same: none of them breaks the body in one night. They each make the repair ledger run at a deficit, quietly, until it doesn't. Fixing my sleep is the highest-leverage item on my list right now.
Sources: Xie et al., "Sleep drives metabolite clearance from the adult brain," Science 2013, https://doi.org/10.1126/science.1241224 · Gooley et al., "Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans," JCEM 2011, https://pubmed.ncbi.nlm.nih.gov/21193540/ · Spiegel et al., "Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite," Ann Intern Med 2004, https://pubmed.ncbi.nlm.nih.gov/15583226/ · Williamson and Feyer, "Moderate sleep deprivation produces impairments in cognitive and motor performance equivalent to legally prescribed levels of alcohol intoxication," Occup Environ Med 2000, https://pubmed.ncbi.nlm.nih.gov/10984335/ · Gangwisch et al., "Short sleep duration as a risk factor for hypertension," Hypertension 2006, https://www.cuimc.columbia.edu/news/lack-sleep-linked-increased-risk-high-blood-pressure
Note: the brain-clearance finding is disputed. A 2024 Nature Neuroscience study in mice found clearance reduced, not increased, during sleep. This post follows Xie et al. 2013, the mainstream citation. https://www.nature.com/articles/s41593-024-01638-y