The Wellness Brew - Issue 19

Sleep is one of the most modifiable inputs to long-term health, and one of the most measurable. Millions of Americans now sleep with a wearable on their finger, wrist, or under their mattress. The question is what to do with all that data. This week's Deep Dive covers the four biomarkers that carry almost all the signal and the six interventions that meaningfully move them.

But first, we filtered the noise — here's what's worth knowing this week.

A May 2026 study published in Cell Host & Microbe by researchers at Harvard T.H. Chan School of Public Health analyzed stool samples from 354 women an average of 12 years after a colorectal adenoma (a precancerous polyp) was removed. The findings: microbial and metabolic profiles associated with colorectal cancer were still detectable more than a decade after the polyp itself was gone. Importantly, this does not mean the polyp removal caused the disruption. The more likely interpretation is that the underlying gut environment that produced the polyp in the first place (diet, lifestyle, chronic exposures) persisted well after the physical polyp was removed. The clinical implication is that removing the polyp addresses the symptom, not the root cause. For anyone with a family history of colorectal cancer or a history of adenomas, the long-term protection likely comes from what you eat and how you live, not from the surveillance colonoscopy alone. — Nogal et al., Cell Host & Microbe, 2026.

A 2019 study published in JAMA Network Open followed 1,104 male firefighters for 10 years and measured how many push-ups they could do at baseline. The men who could do 40 or more push-ups had a 96 percent lower risk of cardiovascular events during the follow-up period compared to those who could do 10 or fewer. Push-up capacity was actually a stronger predictor than treadmill test results. The important framing is what the number represents rather than the number itself. You do not accidentally do 40 push-ups. The people who could do 40 were also the people who exercised regularly, maintained lower body weight, and had better metabolic markers. The push-up count is a signal of the underlying pattern, not the cause. The takeaway is not that everyone needs to train up to 40 push-ups tomorrow. It is that the composite of habits that produces 40 push-ups is what protects the cardiovascular system. — Yang et al., JAMA Network Open, 2019.

Mamavation, an environmental wellness organization that commissions independent lab testing, sent 11 popular avocado oil brands to an EPA-certified laboratory and found phthalates in every single sample. Brands tested included Primal Kitchen, Chosen Foods, Trader Joe's, and Kroger's Private Selection. Levels ranged from 76 parts per billion at the low end to 56,808 ppb at the high end. The exact source of the contamination is not fully clear, but the leading hypotheses involve plastic tubing during processing and PVC-based food contact materials. Phthalates dissolve into fats, which makes oil-based products particularly susceptible to trace contamination from any plastic in the supply chain. The problem is not exclusive to avocado oil. Similar testing has found phthalates in every olive oil, coconut oil, and ghee sample tested. The practical takeaway: perfectly avoiding phthalates in cooking oils is close to impossible right now. Choosing the lowest-contamination brands and reducing overall exposure where you can is the realistic approach. — Mamavation, Avocado Oils Tested for Phthalates, 2024.

Millions of Americans now sleep with a wearable on their finger, wrist, or under their mattress. The Oura Ring, the Whoop band, the Apple Watch, the Garmin, the 8Sleep Pod, etc. Each collects unprecedented data overnight. Sleep duration, heart rate, HRV, deep sleep, REM, temperature, breathing rate, sleep stages, wake events. The dashboards are dense. The proprietary composite scores (Oura Readiness, Whoop Recovery, Garmin Body Battery) roll dozens of variables into a single number that is easier to glance at and harder to act on.

The result is a lot of data and not a lot of change in how people actually sleep.

Four biomarkers carry almost all of the weight and the composite scores are downstream of these four. Everything else on your wearable is noise that will not meaningfully improve your health. Understanding what the four are, what a good number looks like, and specifically what moves them is the entire game.

1. Sleep duration. The foundation. Multiple large cohort studies have established a U-shaped relationship between sleep duration and mortality, with the lowest risk landing at approximately 7 hours per night.[1] Anything under 7 hours consistently is a red flag for cardiovascular disease, dementia, type 2 diabetes, and depression. Anything over 9 hours consistently is worth investigating (often a signal of undiagnosed sleep apnea or another underlying condition). The 7 to 8 hour range is the target.

Sleep duration is the biomarker every wearable measures most accurately. It is also the one where the intervention is most straightforward. If you are consistently under 7 hours, the answer is not a better mattress or a supplement. The answer is going to bed earlier.

2. Resting heart rate during sleep. Underappreciated and highly informative. Chronically elevated resting heart rate during sleep is an independent predictor of all-cause and cardiovascular mortality, with elevated risk comparable to smoking, hypertension, or dyslipidemia.[2] Most people focus on daytime resting heart rate, but the nighttime measurement is more consistent because you are actually at rest and the number is not contaminated by caffeine, stress, or movement.

The absolute number varies by individual, but the direction of movement over months matters as much as the number itself. If your sleeping RHR is drifting up over several months, something is happening (stress, poor recovery, undertreated illness, alcohol use, weight gain). If it is trending down, your cardiovascular fitness is improving. For most healthy adults, sleeping RHR ranges from the low 50s to low 70s. Elite athletes can often dip into the 30s. Anything consistently above 75 warrants attention.

3. Heart rate variability (HRV) during sleep. The most sensitive of the four biomarkers, and the one where the evidence base has strengthened significantly in the last few years. A 2022 meta-analysis published in Neuroscience & Biobehavioral Reviews pooled 32 studies covering 38,008 participants and found that lower HRV was a significant predictor of higher all-cause mortality across ages, sexes, continents, and populations.[8] The finding held in healthy adults and in patients with existing cardiovascular disease. The direction is clear. Higher HRV is associated with lower mortality risk. Lower HRV is associated with higher mortality risk. HRV is now recognized as a genuine biomarker of cardiovascular and overall health, not just a proxy for stress or recovery.

The individual baseline still matters more than the absolute number. HRV varies dramatically between people (some run 30 ms, some run 100 ms) and the range that is "normal" depends heavily on age, sex, and fitness level. But the direction of your personal trend is what matters. A 20 percent drop from your baseline is a signal that something is stressing your system. HRV also has the shortest response time of the four biomarkers, meaning it will show the effect of a poor night’s sleep within 24 to 48 hours. It can also predict when you’re getting sick. The short response makes HRV extremely useful for behavior change.

4. Sleep timing consistency. The most underdiscussed and most immediately actionable. A large-scale UK Biobank longitudinal study found that higher night-to-night variation in sleep timing was associated with increased risk of myocardial infarction and stroke, independent of total sleep duration and independent of genetic risk.[3] In other words, going to bed at 10 PM one night and midnight the next is measurably harmful even if you sleep 7 hours both nights.

The consistency of the wake time matters more than the consistency of the bed time. The body responds most strongly to the anchor of when you get up and see light. If you wake at 6 AM every day and let your bedtime float based on when you get tired, you are doing this right. If your wake time swings by 2 hours between weekdays and weekends, you are creating what researchers call "social jet lag," which produces the same cardiovascular effects as actual jet lag but happens every week.

Deep sleep and REM percentages get a lot of attention on wearables because they sound important. They are important. But they are not as directly controllable as the four biomarkers above, and chasing specific percentages on a wearable is the wrong optimization target for most people.

What they do. Deep sleep is where physical restoration happens, including growth hormone release, immune consolidation, and cardiovascular recovery. REM is where cognitive and emotional processing happen, including memory consolidation and mood regulation. Both matter. Target ranges are well-established (roughly 13 to 23 percent of total sleep for deep sleep and 20 to 25 percent for REM).

Why they are harder to control. The percentages shift with age (deep sleep declines significantly after 40 in most people, regardless of what they do). They vary within an individual night to night for reasons that are not always clear. Wearable accuracy also varies by device and by stage, which means the specific percentage your device reports may not match what a lab would measure.

What actually moves them. The interventions that improve deep sleep and REM are largely the same as the ones that improve the four biomarkers above. Cooler bedroom, reduced alcohol, consistent timing, less late-evening light. If you focus on the upstream inputs, deep sleep and REM improve as a downstream effect. If you focus on the percentages themselves, you will chase noise.

The simpler framing: track deep sleep and REM as informational metrics that give you a sense of your sleep architecture. Do not treat them as optimization targets.

Six specific inputs, ordered by leverage, with the evidence for each.

1. Reduce alcohol, especially within 3 hours of bed. The single biggest lever most people have and the intervention where wearable data is most useful. Alcohol suppresses HRV, elevates sleeping RHR, and disrupts REM sleep in a measurable dose-response pattern.

A study of 42,086 Finnish adults using continuous heart rate monitoring quantified the effect precisely. Low doses (roughly one drink) reduced parasympathetic recovery during the first hours of sleep by about 9 percent. Medium doses (2 to 3 drinks) reduced it by 24 percent. Higher doses reduced it by more than 30 percent.[4] More recent Eight Sleep data on thousands of users showed that even one standard drink increases sleeping HR by roughly 2 percent and lowers HRV by 3 to 4 percent. Four drinks produces a 7 percent HR increase and a 15 percent HRV drop that lingers for the entire night.

Two things worth knowing. First, most people cannot perceive the difference in how they feel between a drinking and non-drinking night. The subjective experience of sleep quality often stays the same while the biomarkers show a substantial cost. Second, the effect is dose-responsive and lingering. HRV can take 2 to 4 days to fully recover from moderate drinking, which means a Friday night out affects your Monday morning readiness.

2. Get the bedroom cold. Temperature is the second-most-actionable intervention. Multiple studies show that a bedroom in the 65 to 68°F range improves deep sleep, REM, and HRV. A 2024 study on 54 adults using an active temperature-regulated mattress found that cooler temperatures in the first half of the night increased deep sleep by 22 percent and REM by 25 percent compared to non-controlled sleep.[5] 

The reason temperature has such a big impact is that the body’s core temperature declines during sleep and then rises to wake you up. A bed that’s too warm tells the body it’s time to wake up. You do not need an expensive smart mattress to get most of the benefit. A cheap fan and turning down the thermostat capture the majority of the effect. 

3. Anchor your wake time. Same wake time seven days a week, including weekends. This is the single highest-leverage no-cost intervention available. The consistency of the wake time is what stabilizes the circadian rhythm and drives the compounding benefits.

Let the bedtime float based on when you get tired. If you get to bed late one night, still get up at your normal wake time. Yes, you will be tired the next day. Your circadian rhythm will thank you for it, and your baseline sleep quality will improve within a week or two.

4. Get morning sunlight within 30 minutes of waking. Ten to fifteen minutes of direct outdoor light exposure anchors the circadian rhythm and improves nighttime sleep quality. The intensity difference between outdoor and indoor light is roughly 100 times, and the circadian system responds to the intensity, so through a window does not count.

This is one of the strongest levers for anchoring your circadian rhythm. It is also one of the hardest to execute for anyone in the northern hemisphere in winter, anyone with a pre-dawn schedule, or anyone whose morning routine does not naturally include outdoor time. If you can get outside for 10 to 15 minutes shortly after waking, do it. A cup of coffee on the porch is the entire protocol. If you cannot, do not stress. The other five interventions carry most of the load.

5. Stop eating three hours before bed. A February 2026 study followed adults who extended their overnight fast by three hours for eight weeks and found meaningful improvements in overnight cardiovascular metrics: lower sleeping heart rate, higher HRV, 12 percent lower nighttime cortisol, better morning glucose regulation.[6] Total calories did not change. The timing of the calories did.

The practical framing: if you go to bed at 10 pm, stop eating by 7 pm. If you go to bed at 11, stop by 8. For people with flexible schedules this is straightforward. For anyone getting home from work at 7 with kids to feed and a family dinner to hold together, the 3-hour window is often impossible. If that is your reality, treat this as an aspirational target for the nights that allow it (weekends, days without evening obligations) rather than a nightly rule. The consistency of the other interventions matters more than perfect execution of this one.

6. Reduce evening and overnight light exposure. A 2022 Northwestern study published in PNAS established that even moderate ambient light during sleep (100 lux, roughly what you get from a bright hallway light) elevated nighttime heart rate, decreased HRV, and increased next-morning insulin resistance compared to sleeping in a dim room.[7]

Close the blinds. Cover the LED lights on electronics. Consider blackout curtains if streetlights or morning sun come through your windows. This is one of the cheapest, easiest, and most under-utilized sleep environment interventions available.

Sleep is where marginal gains compound fastest. A single night of better sleep produces measurable next-day benefits. A month of consistent sleep produces cumulative cognitive, metabolic, and cardiovascular benefits. A decade produces meaningfully different aging trajectories.

The wearable is not the point. The wearable is a tool for understanding what actually moves the needle for your specific physiology. Track the four biomarkers. Apply the six interventions. Once you know what works for you, the device becomes optional. 

Four things you can do this week:

1. Track four numbers and ignore the rest.
Sleep duration, sleeping resting heart rate, HRV, and sleep timing consistency. Everything else on your wearable dashboard is downstream of these four. If your composite sleep score is trending down over weeks, look at the underlying four. If it fluctuates day to day, ignore it.

2. Reduce alcohol.
The single highest-leverage intervention for sleep biomarkers. Cutting from three drinks per week to one will produce more measurable improvement in HRV, RHR, and next-day recovery than any supplement, gadget, or protocol. Every wearable now tracks this and the response is unambiguous. If you drink, at least stop drinking three hours before bed.

3. Get the bedroom below 68°F.
Turn down the thermostat. Add a fan. Open a window if the outside temperature allows. Temperature is the most under-utilized sleep lever. 

4. Anchor the wake time seven days a week.
Same wake time weekdays and weekends, within 30 minutes. Bed time can float based on when you get tired. The consistency of the wake time is what stabilizes the circadian rhythm and drives the compounding benefits. This is the single highest-leverage no-cost intervention available.

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Wellness, filtered.

The Wellness Brew

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Disclaimer: The Wellness Brew is for informational purposes only and does not constitute medical advice. The content published here is not intended to diagnose, treat, cure, or prevent any disease or health condition. Always consult a qualified healthcare professional before making any changes to your diet, supplement routine, or lifestyle.