Does ashwagandha actually lower cortisol and raise HRV?

What the evidence actually shows, claim by claim

Ashwagandha (Withania somnifera) gets marketed as an all-purpose "adaptogen" for stress, sleep, and recovery, but those three claims don't rest on equally strong evidence. Separating them is the point of this article.

The cortisol evidence, and why it's real but not clean

The most frequently cited single trial is a 60-day study of 600 mg/day of KSM-66 ashwagandha root extract, which reported morning cortisol falling roughly 28% in the ashwagandha group versus about 8% in the placebo group — a meaningful between-group difference. That result sits inside a broader, more reassuring pattern: a 2026 systematic review and meta-analysis pooling 23 randomized, placebo-controlled trials (1,706 participants total) found a statistically significant cortisol reduction, with some evidence of a dose-response relationship tied to withanolide content (the compound class ashwagandha extracts are standardized against). A second, independently conducted 2025 meta-analysis of 7 Indian RCTs (488 participants) reached the same cortisol conclusion.

Two things temper how much weight to put on that: first, individual trial sample sizes are mostly small — commonly under 100 participants — so pooled results carry more statistical power than any single study the headlines are usually drawn from. Second, the field has a real industry-funding problem. The most widely cited KSM-66 trial was published with promotional involvement from Ixoreal Biomed, the manufacturer of KSM-66, which has funded or co-authored multiple ashwagandha studies. Industry funding doesn't automatically make a result false, but it's a documented source of inflated effect sizes across nutrition research generally, and it's a reasonable basis for shaving your personal expectations down from "28% drop" toward "a real but probably smaller effect."

Why the HRV claim doesn't hold up the same way

This is the gap that matters most if you're deciding whether ashwagandha is worth taking because of your HRV number. Cortisol and HRV are physiologically linked — chronically elevated cortisol is associated with reduced parasympathetic (vagal) tone, which is part of what HRV measures — so "lowers cortisol" getting rounded up to "therefore raises HRV" is an intuitive leap. But a direct search of the clinical trial literature on ashwagandha and HRV turns up essentially no published randomized trial that measured HRV as a primary or even secondary outcome. The trials behind the cortisol finding measured cortisol, DHEA-S, testosterone, and anxiety/stress questionnaires — not HRV. A registered trial using wearable-collected HRV data in women was still recruiting participants at time of writing, which tells you the question is considered open enough to be worth a dedicated study, not settled enough to cite as fact.

If you see a specific HRV percentage attributed to ashwagandha — "improves HRV by X%" — treat it the way you'd treat an unsourced claim anywhere else in supplement marketing: as probably derived from the cortisol finding rather than measured directly, until a primary trial says otherwise.

Root extract vs. leaf-and-root extract: what the labels mean

Ashwagandha supplements aren't interchangeable, and most of the trial evidence above is specific to one or two standardized extracts rather than "ashwagandha" generically — a label that just says "ashwagandha root powder" with no standardization is a different product than what most trials tested.

Research focus differs too: KSM-66 has mostly been studied for stress, cortisol, endurance, and cognitive performance, while Sensoril shows up more often in sleep-quality and metabolic/body-composition trials. If a product doesn't name a standardized extract or withanolide percentage on the label, there's no trial evidence it maps onto, regardless of what the front of the bottle claims.

Safety: thyroid, liver, and who should skip it entirely

Ashwagandha's "natural herb" framing undersells a real, documented safety signal that's emerged over the past few years — this isn't a hypothetical caution, it's the basis for active regulatory review in multiple countries.

• Thyroid. Ashwagandha can raise circulating thyroid hormone levels. That's a meaningful consideration if you have an existing thyroid condition or take thyroid medication (such as levothyroxine), where an unplanned hormone shift could push you toward thyrotoxicosis symptoms.
• Liver. The Netherlands' pharmacovigilance centre flagged a safety signal in September 2023 after four case reports of hepatitis, abnormal liver function, cholestasis, and jaundice in people who had used ashwagandha for 3-10 months. A 2023 case series published in Hepatology Communications described a similar cholestatic injury pattern, generally reversible after stopping. Causality hasn't been definitively proven in any individual case, and product-quality or contamination issues remain a live possibility — but the pattern was consistent enough that Germany's BfR (2024) and the UK Food Standards Agency now advise people with existing liver disease to avoid ashwagandha supplements, and Denmark banned it in food supplements outright in 2023.
• Pregnancy and breastfeeding. Multiple regulators advise against use here, based on the herb's known endocrine effects and a lack of robust human pregnancy-safety data — this one isn't close enough to call a personal judgment, so skip it.

None of this means ashwagandha is dangerous for a healthy adult with no thyroid or liver history taking it at standard doses for a limited period. It does mean "it's just an herb" undersells the actual regulatory picture in 2026, and it's a reasonable supplement to bring up with a doctor rather than self-start if any of those categories apply to you.

A way to find out whether it's actually moving your own numbers

Given how thin the direct HRV evidence is, your own recovery trend is a more honest test than trusting a marketed percentage either way:

1. Log a 1-2 week baseline first, without changing anything else, using your current average HRV, resting heart rate, and how stressed you'd rate yourself day to day.
2. Pick one standardized extract and stick with it — KSM-66 at 300-600 mg/day is the one with the most trial support; don't switch brands or extracts mid-trial, since dose and standardization both vary.
3. Give it the trial length the research actually used — most positive cortisol trials ran 8-12 weeks, not days, so a one-week trial isn't long enough to judge by.
4. Hold training load, alcohol, and sleep schedule roughly steady for the comparison window, since all three move HRV more reliably than a supplement is likely to.
5. Compare weekly averages, not single days, and be honest with yourself if nothing's moved after 8-12 weeks — that's a legitimate outcome given how thin the HRV-specific evidence is.

Vita's recovery score tracks HRV and resting heart rate against your own rolling baseline rather than a population average, which is the comparison this kind of self-trial actually needs, and its AI coach can help you reason through whether a shift lines up with the supplement or with something else that changed in the same window — training load, a stressful week, or a sleep disruption. If you're also evaluating other supplements against your wearable data, our guides on creatine and HRV/sleep and magnesium and sleep cover the same self-testing logic for two commonly stacked alternatives.

The bottom line

Ashwagandha's cortisol-lowering effect is the best-supported part of its reputation — two independent meta-analyses of randomized trials agree on it, even after accounting for small sample sizes and real industry funding in the most-cited individual study. Its reputation as an HRV booster is a different story: that claim appears to be an inference from the cortisol finding rather than something any published trial has directly measured, so treat it as unproven rather than disproven. If you decide to try it, KSM-66 at 300-600 mg/day for at least 8-12 weeks matches what the trial evidence actually tested, and anyone with thyroid disease, liver disease, or who is pregnant or breastfeeding should treat the safety signals as a reason to check with a doctor first, not an afterthought.