Physiological Changes During Sleep

One week of short sleep in otherwise healthy adults. Not "sleep deprived" by strict definition. Just 4-5 hours a night. Testosterone dropped 15%. Insulin sensitivity dropped 20%. Muscle protein synthesis dropped 19%. Hunger hormones rose 28%. Cortisol rose 51%. These aren't the only systems affected. They're just some of the ones that have been measured in controlled settings. No supplement, no diet hack, no training program (crazy claim, I know, but you can't outrain poor sleep...) outperforms sleep at keeping systems "online". References: Leproult & Van Cauter, JAMA, 2011. Buxton et al., Diabetes, 2010. Zuraikat et al., Diabetes Care, 2024. Spiegel et al., Lancet, 1999. Saner et al., J Physiol, 2020. Spiegel et al., Ann Intern Med, 2004.

Sleep is not just rest. It is active maintenance for your brain. Research shows that during sleep, the brain activates the glymphatic system, a network responsible for clearing metabolic waste and toxins that accumulate throughout the day. This process is significantly more active during deep sleep, when brain cells shrink slightly to allow fluid to flow more efficiently and wash away byproducts like beta amyloid, a protein linked to Alzheimer’s disease. When sleep is cut short or disrupted, this clearance process is reduced, allowing waste to build up over time. Studies consistently link poor sleep with cognitive decline, impaired memory, and increased neurological risk. The brain cannot perform this level of cleanup while you are awake. Sleep is not downtime. It is essential biological housekeeping that protects long term brain health. (Science Magazine; Xie et al. 2013; National Institutes of Health)

Most people worry about their heart during the day. But the real damage, or protection, happens at night. The most protective thing your heart does each day is slow down and reset at night. During deep sleep, blood pressure should fall by 10–20%. Heart rate should soften. Inflammation should quiet. This is called nocturnal dipping. And when it doesn’t happen, cardiovascular risk rises, quietly, progressively, invisibly. People with: – “Normal” daytime blood pressure – Good cholesterol – Decent fitness …but fragmented or short sleep Often show: – Elevated night-time blood pressure – Higher inflammatory markers – Early vascular ageing, years before symptoms appear Sleep isn’t recovery from stress and repairs for the body. It’s when the cardiovascular system repairs stress-related damage. And this is where most advice falls short, so let me make it practical. 3 sleep metrics that actually matter for your heart 👇🏻 1. Regularity beats duration Seven hours at random times is not the same as seven hours consistently. Irregular sleep keeps your nervous system in “alert mode”, even at night. → Aim for a fixed wake-up time first. The rest follows. 2. Your breathing during sleep matters more than you think Snoring, mouth breathing, or waking unrefreshed are not benign. They often mean repeated oxygen drops → adrenaline surges → cardiac strain. If blood pressure is hard to control, always ask: how am I breathing at night? 3. The heart needs a positional advantage For some hearts, especially those with breathing concerns, sleep position can affect comfort and breathing patterns. Right-side sleeping or slight head elevation can reduce overnight stress in selected patients. This is not a universal rule, it’s personal physiology. A simple heart-protective sleep checklist I give patients: – Same wake-up time, even on weekends – No caffeine after midday – Screens off at least 60 minutes before bed – Dark, cool, quiet bedroom – If using a wearable: check whether heart rate and blood pressure actually drop overnight If they don’t, that’s not a mindset issue. That’s a signal. Here’s the key message I want people to understand: You don’t optimise sleep to feel less tired. You optimise sleep to reduce cardiovascular wear and tear. Sleep is when the heart: • lowers pressure • repairs vessels • resets rhythm • dampens inflammation Ignore it, and everything else works harder, with less return. Your heart doesn’t need more effort. It needs better nights.

How sleep regulates cellular ageing (and why recovery is non-negotiable for longevity) Sleep is not downtime. It is an active biological process that governs cellular repair, metabolic balance, and ageing speed. The problem we keep overlooking In modern life, sleep is often the first thing we sacrifice. Late nights, early starts, screens, caffeine, stress. Clinically, we see the consequences daily. Poor sleep is not just fatigue. It is a signal disruption that cascades down to the cellular level. When sleep becomes fragmented or misaligned, the body loses its primary repair window. What changes at a cellular level Sleep is when circadian rhythms synchronise cellular activity. When sleep is inadequate or mistimed: • DNA repair pathways are suppressed • Hormonal rhythms flatten • Mitochondrial efficiency declines • Oxidative stress accumulates • Cellular cleanup processes slow These processes do not fail suddenly. They drift out of rhythm. The downstream biological effects Over time, this cellular dysregulation manifests as: • Insulin resistance • Chronic low-grade inflammation • Immune dysfunction • Cognitive slowing • Accelerated biological ageing This is how disrupted sleep translates into disease vulnerability. Why mitochondria and circadian rhythm matter Mitochondria respond directly to circadian cues. They function best when sleep timing, light exposure, and energy demand are aligned. When circadian rhythm is disrupted: • Energy production becomes inefficient • Cellular stress signalling increases • Repair shifts toward damage control Ageing accelerates not from lack of sleep alone, but from loss of biological timing. The restorative window we must protect Quality sleep restores balance. During deep and aligned sleep: • DNA repair is upregulated • Hormones recalibrate • Mitochondria renew • Autophagy clears damaged components • Inflammation resolves This is not optional recovery. It is cellular maintenance. What actually works in practice Longevity does not require perfect sleep hygiene. It requires consistency and alignment. Clinically effective foundations include: • Regular sleep and wake times • Morning light exposure • Reduced late evening stimulation • Caffeine timing awareness • Respecting recovery as treatment Small corrections restore powerful biology. Ageing is not driven only by damage. It is driven by missed repair opportunities. Sleep is where those opportunities live. If you care about longevity, start treating sleep as cellular medicine. #LifestyleMedicine #LongevityMedicine #SleepScience #CellularHealth #PreventiveCare

I have been teaching for years that REM sleep is when the brain finds emotional balance, recalibrating the physiological reaction to yesterday’s emotionally difficult moments. People usually just nod politely. This week, a new study adds more evidence. Researchers experimentally fragmented REM sleep in healthy adults without diminishing total sleep time (a really cool procedure, by the way!). Participants still slept about seven hours at normal efficiency. The researchers then tested whether they could physiologically habituate to emotional stimuli the next day. Fragmenting REM sleep impaired overnight habituation of a heart rate measure of stress called the cardiac deceleration response (at 24 hours and again at 48 hours). The degree of impairment tracked alpha-power intrusions over parieto-occipital regions during the stimulated REM sleep, tying a behavioral effect to a specific cortical signature. The leaders among you will recognize the feeling, I'm sure. You wake up and the meeting from yesterday is still lit up in your chest, and you can't seem to find the off switch. The thing to understand here is that REM fragmentation is invisible. You managed to sleep for seven hours. Maybe your sleep tracker even reported 87 percent. You feel tired but nothing alarming, and yet your nervous system is carrying Friday’s unprocessed emotional load into Monday. Common culprits for REM fragmentation include alcohol in the second half of the night when REM is heaviest, late caffeine, untreated mild sleep apnea, a baby monitor, a partner who moves a lot, a phone that buzzes, and, of course, stress itself. Small perturbations like these have real consequences. If your emotional resilience has felt disproportionately off lately, the explanation may have less to do with character and more to do with sleep continuity. We're actively working in my lab to ameliorate this with targeted reactivation of the relaxation response during sleep. Paper: Viselli et al., Sleep, April 2026. DOI: 10.1093/sleep/zsaf409

One night of poor sleep increases amyloid-beta in your brain by 5%. Not one year. Not one month. One night. Researchers at the NIH used PET imaging to measure amyloid accumulation after a single night of sleep deprivation. The increase was significant and measurable. Why this matters: Amyloid-beta is the protein that accumulates in the brains of Alzheimer's patients. Your brain clears it during deep sleep through the glymphatic system, a waste-removal network that activates primarily when you're in slow-wave sleep. Skip that sleep and the waste doesn't get cleared. Now multiply that by years of poor sleep. What the research tells us: People who consistently sleep less than 6 hours per night have a 30% higher risk of developing dementia compared to those who sleep 7-8 hours. The relationship between sleep and dementia is bidirectional: Poor sleep accelerates amyloid accumulation ↳ Amyloid accumulation disrupts sleep quality ↳ Which accelerates more amyloid accumulation ↳ The cycle feeds itself Sleep apnea makes it worse: Roughly 80% of sleep apnea cases are undiagnosed. In my dementia clinic, I test almost every patient for it. When I find it and treat it, cognitive function often improves within months. Sleep apnea causes intermittent oxygen deprivation throughout the night. Every episode is a small assault on brain tissue. What I tell patients: Sleep is not optional recovery time. It's when your brain does its most critical maintenance. If you consistently wake up tired, if your partner says you snore or stop breathing, if you need caffeine to feel functional by 10am, get a sleep study. Treating sleep apnea is one of the highest-yield interventions in dementia prevention. It's also one of the most underdiagnosed. Simple sleep hygiene steps that protect your brain: Consistent sleep and wake time, even weekends ↳ Dark, cool room (65-68 degrees) ↳ No screens 30 minutes before bed ↳ Limit caffeine after noon ↳ Alcohol disrupts sleep architecture even if it helps you fall asleep Your brain cleans itself at night. Let it. ⁉️ How many hours of sleep do you consistently get? Be honest. I'll go first: I aim for 7.5 and usually hit 7. ♻️ Repost if you think sleep is the most underrated health intervention 👉 Follow Reza Hosseini Ghomi, MD, MSE for evidence-based brain health from the clinic Citations: hokri-Kojori E et al. β-Amyloid accumulation in the human brain after one night of sleep deprivation. Proceedings of the National Academy of Sciences. 2018. Sabia S et al. Association of sleep duration in middle and old age with incidence of dementia. Nature Communications. 2021.

Sleep isn’t laziness. It’s your body’s most powerful repair system. Most people treat sleep like a luxury. Something to squeeze in after work is done. after emails are answered. after one more episode. I used to think the same. But the more I studied health, performance, and longevity, the clearer it became: Sleep isn’t passive. It’s one of the most active recovery processes your body has. While you sleep: → Your brain clears toxins through the glymphatic system → Cells repair damaged tissues → Hormones rebalance → Memories consolidate → Stress regulation improves → Your heart and metabolism reset In other words… Your body is doing some of its most important work when you’re not awake. Yet many high performers still sacrifice sleep first. The irony? When sleep suffers: → Focus drops → Emotional stability declines → Recovery slows → Decision-making worsens Sleep isn’t time lost. It’s the foundation that makes everything else work better. So instead of asking: “How little sleep can I get away with?” A better question is: “How much better could I perform if I protected my sleep?” ♻️ Share this with someone who treats sleep like an afterthought. 🔔 Follow Sanjiv Beri for practical insights on sleep, longevity, and high performance.

🚨 Just listened to the latest WHOOP podcast with our very own Gina R. Poe on all things REM, deep sleep, and the brain’s overnight reset. Let’s discuss some of the key takeaways: Your brain isn’t just resting during sleep—it’s 𝘴𝘵𝘳𝘢𝘵𝘦𝘨𝘪𝘤𝘢𝘭𝘭𝘺 𝘤𝘭𝘦𝘢𝘯𝘪𝘯𝘨, rebalancing, and reorganizing. Missing the 𝘧𝘪𝘳𝘴𝘵 𝘩𝘢𝘭𝘧 of the night? You’re likely missing crucial 𝙨𝙡𝙤𝙬-𝙬𝙖𝙫𝙚 𝙨𝙡𝙚𝙚𝙥, when your brain clears out metabolic waste and resets synaptic strength. Think REM sleep is “lighter”? Think again. This is when 𝙚𝙢𝙤𝙩𝙞𝙤𝙣𝙖𝙡 𝙧𝙚𝙥𝙧𝙤𝙘𝙚𝙨𝙨𝙞𝙣𝙜, memory integration, and even trauma healing happen. REM is the mind’s editing room. The 𝙇𝙤𝙘𝙪𝙨 𝘾𝙤𝙚𝙧𝙪𝙡𝙚𝙪𝙨 (𝙇𝘾)—a tiny blue nucleus in the brainstem—plays a huge role in regulating arousal and attention. But it needs to 𝘨𝘰 𝘴𝘪𝘭𝘦𝘯𝘵 during REM so emotional memories can be safely reprocessed without triggering a stress response. If the LC stays active? Emotional memories might not be properly defused. The 𝘀𝗲𝗾𝘂𝗲𝗻𝗰𝗲 of sleep stages matters. NonREM primes the brain, REM puts it all together. Disrupt that rhythm, and you don’t just lose hours—you lose function. And yes—𝗲𝘅𝗲𝗿𝗰𝗶𝘀𝗲 𝗵𝗲𝗹𝗽𝘀. Even our daily “office-stair challenge” (4 floors up, followed by 5 minutes of breathless silence before lunch 🍜) contributes to better sleep architecture over time. Still beyond excited (and a little in awe) to have Gina as part of our team. Her work helps shape how we think about sleep—and why we’re building what we’re building. 👉 Check out the full episode to dive into all the ins and outs. Link in comments.

That isn’t a scare tactic. If you choose to sleep 6 hours instead of 8, you are choosing to age 25% faster. During my time as a physician for the Navy SEALs, I witnessed a baffling paradox. I would have operators walk into my clinic who were physically elite — “jacked,” training relentlessly, and eating perfectly. But when I ran their labs, their bloodwork looked like that of a 55-year-old, overweight, pre-diabetic man. They had low testosterone, high inflammation, and insulin sensitivity issues. Why? They were suffering from “underecovery.” In our culture, we often treat sleep as a passive state — a “time out” from productivity. We wear our sleep deprivation like a badge of honor. But sleep is active biological work. Here is the science we often ignore: 📉 The 25% Deficit: The sole purpose of sleep is to repair the damage from today and prepare for tomorrow. It takes roughly 8 hours to recover from being awake for 16. If you cut that by 2 hours (25%), you don’t just “feel tired.” You lose 25% of your total recovery capacity. 🔥 You Become Catabolic: To bridge that gap, your body releases stress hormones to keep you going. These hormones are catabolic — meaning they break you down. You literally start using your own body as a fuel source to survive the day. 🧠 The “Hormonal” Split: First half of the night: 98% of your testosterone and 100% of your growth hormone is released here. Second half of the night: Your brain cleans out metabolic waste and consolidates memory. When you cut sleep short or use alcohol/sedatives (which destroy REM and Deep Sleep), you are skipping the body’s only opportunity to repair itself. We need to stop viewing sleep as the “Fourth Pillar” of health. It is not a pillar. It is the foundation. Exercise, nutrition, and stress management all sit on top of it. If the foundation cracks, the entire structure of your performance collapses. Stop “hustling” yourself into an early grave. Prioritize the foundation. 👇 How many hours are you actually averaging right now? Be honest in the comments.

🧊 Stay cool to stay asleep. Yesterday, we talked about how your body has to cool down to power down. Once you’re powered down, the next challenge is staying asleep—and that depends on keeping your core temperature low and stable. 🌙 Why it matters: During deep sleep, your body continues to shed heat. Even small rises in temperature can pull you out of slow-wave sleep and into lighter, more fragmented stages. 📊 The science: • Best ambient temperature: 60–67°F (15–19°C) for most adults. • Warmer rooms shorten REM, lower HRV, and reduce next-day alertness. • Perimenopause: Hormonal fluctuations (especially declining estrogen) disrupt the hypothalamic thermostat, narrowing the normal thermal buffer and triggering night sweats/hot flashes that destabilize core-temperature regulation during sleep. 🔥 Why late-night habits raise your core temperature: • Heavy meals: The thermic effect of food increases metabolic heat during digestion. • Intense workouts: Elevated muscle metabolism and circulation delay cooling. • Alcohol: Causes blood vessels to dilate, making you feel warm initially, but later triggers rebound heating and restless sleep. 🛏️ Practical cool-downs: • Set your thermostat before bed to cool the room. • Choose breathable sheets—cotton, linen, or bamboo. • Keep air circulating with a fan or cracked window. • If you run warm, consider smart cooling tech like the BedJet (airflow), ChiliPad / Dock Pro (water-cooled pad), or Eight Sleep Pod (smart temperature control and HRV tracking). 💡 You fall asleep by cooling down. You stay asleep by staying cool. 💭 Have you noticed how just a few degrees’ difference can change your sleep quality? #SleepOptimization #Recovery #Performance #HRV #RestorativeSleep #CircadianHealth #Menopause #Longevity #MetabolicHealth #Biohacking #PhysicianHealth