3 Longevity Studies Worth Your Time: NMN, Zone 2 & Sleep Debt
We read the research so you don't have to. NMN and biological age, zone 2 cardio and metabolic flexibility, and the real cost of chronic sleep debt — plain-English breakdown.
We read the research so you don't have to — but we always link the source so you can. This curated roundup covers three studies that caught our attention on NAD+ metabolism, zone 2 cardio, and sleep debt. Here's what they found and why each one matters.
1. NMN Supplementation and Biological Age Markers in Middle-Aged Adults
Why we're reading it: NMN (nicotinamide mononucleotide) has been one of the most-hyped longevity supplements of the past decade based largely on animal studies. Human data has been thin. This adds to the growing — if still small — human evidence base.
What it found: A 2023 randomized controlled trial published in GeroScience found that 12 weeks of NMN supplementation (250mg and 500mg doses) increased blood NAD+ levels significantly and was associated with improvements in physical performance measures in older adults. No significant adverse effects were observed.
The caveat: Small sample size (n=80), short duration, and physical performance as a proxy for biological aging is imperfect. NAD+ levels increased, but whether that translates to meaningful longevity outcomes in humans remains an open question.
Why you should read it: If you're considering NMN or NR supplementation, understanding what human data actually exists (and what it doesn't say yet) is important for making an informed decision. Don't buy based on mouse studies.
👉 Read the full study in GeroScience
2. Zone 2 Training: Mitochondrial Density and Metabolic Health
Why we're reading it: Zone 2 cardio — low-intensity aerobic training at roughly 60–70% of max heart rate — has become a central pillar of the longevity training conversation, largely due to work from Dr. Iñigo San Millán. This overview paper helps explain why.
What it found: A review by San Millán and Brooks in Sports Medicine (2022) argues that zone 2 training is uniquely effective at improving mitochondrial function and metabolic flexibility — the ability to efficiently switch between fat and glucose as fuel. Metabolic inflexibility is increasingly recognized as a core driver of metabolic disease, type 2 diabetes, and accelerated aging.
The key insight: Zone 2 specifically targets type 1 slow-twitch muscle fibers and the mitochondrial adaptations within them in ways that higher-intensity training doesn't replicate. This is why elite endurance athletes spend 80% of their training volume at low intensity.
Why you should read it: Most recreational athletes spend too much time in "middle zones" — moderate intensity that's too hard to be recovery and too easy to be high-quality stimulus. Understanding zone 2 will probably change how you structure your cardio.
👉 Read the full review in Sports Medicine
3. Sleep Debt Accumulation and Cognitive Function — It's Not Linear
Why we're reading it: Most people understand that poor sleep is bad. What's less understood is how quickly cognitive deficits compound — and how poorly people self-assess their own impairment.
What it found: Research from the Division of Sleep Medicine at Harvard and the University of Pennsylvania has consistently shown that chronic sleep restriction (6 hours/night) produces cognitive deficits equivalent to two full nights of total sleep deprivation within two weeks — but critically, subjects in these studies rated themselves as only "slightly sleepy." People are notoriously bad at perceiving how impaired they are from sleep loss.
The compounding problem: Unlike a single night of poor sleep (from which most people recover with one good night), chronic sleep debt accumulates in ways that aren't fully reversed by a single recovery night. The impact on decision-making, reaction time, and emotional regulation is significant and largely unperceived.
Why you should read it: If you're regularly running on 6 hours and feel "fine," this research is a useful reality check. Sleep is the highest-leverage longevity and performance intervention available — and most optimization-focused people still undervalue it relative to supplements and protocols.
👉 Read the Van Dongen et al. study on chronic sleep restriction at PubMed Central
The OPX Take
Three consistent themes run through longevity research when you read enough of it: mitochondrial health, sleep quality, and NAD+ metabolism keep appearing as central mechanisms of aging. Zone 2 addresses the first. Sleep addresses the second. NMN/NR potentially addresses the third — though with less human evidence than the other two.
If you're going to prioritize: optimize sleep first, add zone 2 training second. Both are free. Supplements come after the fundamentals are in place.
For practical protocols that implement these principles, see our optimal morning routine guide — it's built around the same morning physiology and cortisol science the zone 2 and sleep research underpin. And if you're exploring thermal protocols for recovery, our cold plunge vs sauna breakdown covers the evidence on two of the most accessible biohacking tools available.
We curate research, summarize in plain English, and always link the original source. We never reproduce content — everything here is our own commentary. Click through to read the papers yourself.