The Executive's Guide to Cortisol, the HPA Axis, and Winding Down

You know the pattern. The day is finally over, the laptop is shut, and your mind is still running the 4pm meeting on a loop. You are tired, but you are also wired. For a lot of high-performing people, the problem isn't a lack of stress. It's that the stress system never seems to fully switch off.

To understand why, and to understand what the research on adaptogens and calming herbs is actually measuring, it helps to look at the machinery underneath. This is a guide to the HPA axis, the daily cortisol rhythm, and the physiology of winding down, written for people who want the evidence rather than the hype. For New Zealanders researching ashwagandha or stress supplements, here is what the evidence actually supports.

What the HPA axis actually does

Your stress response is coordinated by a three-part loop called the hypothalamic-pituitary-adrenal (HPA) axis. When your brain registers a demand (a deadline, a difficult conversation, a red-eye flight) the hypothalamus releases corticotropin-releasing hormone (CRH). That signals the pituitary gland, which in turn signals the adrenal glands sitting on top of your kidneys to release cortisol, the body's primary stress hormone.

Cortisol is not the villain it's often made out to be. In the short term it's genuinely useful: it mobilises glucose for energy, sharpens focus, and dampens processes the body can postpone during a challenge. The system is also self-regulating. Circulating cortisol feeds back to the hypothalamus and pituitary and tells them to ease off, a thermostat-style negative feedback loop that's supposed to return you to baseline once the demand passes.

The trouble is chronic, unrelenting demand. When the signal never lets up, that feedback loop can become less sensitive, and cortisol can stay elevated when it should be tapering. Researchers studying the stress response describe this as a disruption of HPA-axis balance, and it's one of the things clinical trials on adaptogens set out to measure.

The cortisol curve: why timing matters

Cortisol isn't meant to be flat. In a healthy rhythm it follows a diurnal curve: a sharp rise in the first 30 to 45 minutes after waking (the cortisol awakening response), a gradual decline across the day, and a low trough around the middle of the night. That morning peak is part of what gets you out of bed and moving.

For winding down, the evening slope is the part that matters. Ideally cortisol is low and falling as bedtime approaches, clearing the way for the systems that promote sleep. When the curve is blunted or shifted, high in the evening and sluggish in the morning, the subjective experience is exactly that "tired but wired" feeling. Supporting a healthy cortisol rhythm, rather than crushing cortisol altogether, is the more sensible framing, and it's the one the better research reflects.

Adaptogens and ashwagandha: what the withanolides do

Adaptogens are a class of botanicals traditionally used to help the body cope with stress. The best-studied of them is ashwagandha (Withania somnifera), a plant used in Ayurvedic practice for centuries and now the subject of a growing body of randomised, placebo-controlled trials.

The active constituents are a family of steroidal lactones called withanolides. In research settings, standardised extracts are typically characterised by their withanolide content, and it's these compounds that studies associate with effects on the stress response. Mechanistic work suggests withanolides may influence CRH signalling and glucocorticoid sensitivity. In other words, they appear to interact with the same negative-feedback machinery described above, which is a plausible route by which an adaptogen might support the HPA axis in returning toward baseline.

On the clinical side, several randomised controlled trials in chronically stressed adults have measured serum cortisol and validated stress questionnaires before and after supplementation. A number of these trials report reductions in cortisol relative to placebo, alongside improvements in self-reported stress and sleep-quality scores. A 2025 systematic review and meta-analysis noted a consistent signal for cortisol reduction across studies, while finding no significant effect on perceived stress in the pooled analysis. That is a useful reminder that biomarkers and subjective experience don't always move in lockstep. It's also worth noting that ashwagandha is studied as a daily, cumulative-use botanical, not a fast-acting sedative.

For readers who want a single, standardised extract to explore, our Pure Encapsulations Ashwagandha provides a withanolide-standardised root extract of the kind used in this research.

When one herb isn't the whole picture

Ashwagandha is the headline adaptogen, but the stress response is multi-layered, and formulators often combine complementary compounds. A cortisol-support formula typically pairs an adaptogen with other researched ingredients, for example magnolia bark (a source of honokiol and magnolol) and the amino acid L-theanine, both of which have been studied for their interaction with GABAergic signalling and calm, alpha-wave brain states.

The logic is to touch more than one node of the system at once: an adaptogen aimed at the HPA feedback loop, plus compounds studied for the moment-to-moment sense of calm. L-theanine in particular has been researched for its association with alpha-wave activity, an EEG signature of relaxed alertness, which is why it appears in formulas meant to steady rather than sedate. Our Pure Encapsulations Cortisol Calm is built along these lines, combining ashwagandha and rhodiola with magnolia and L-theanine as a single daily formula for those looking to support a healthy stress response.

Winding down without sedation: the GABA pathway

Here's the distinction that matters most for evening use. Feeling calm and being sedated are not the same thing. Sedation dulls you; calm simply turns down the volume on nervous-system arousal. The neurochemistry behind that distinction runs largely through a single molecule: GABA (gamma-aminobutyric acid).

GABA is the brain's principal inhibitory neurotransmitter, the "off switch" that quiets excitatory signalling and lets the nervous system downshift. When GABAergic tone is adequate, the transition from alert to at-ease is smoother. When arousal chemistry dominates into the evening, that transition stalls, which is the physiological version of lying in bed with a racing mind.

Passionflower and the herbal calm tradition

Several traditional calming herbs have been investigated for their interaction with this pathway. The most researched is passionflower (Passiflora incarnata). In laboratory studies, whole passionflower extract has been shown to modulate the GABA system, inhibiting GABA uptake and interacting with GABA-A and GABA-B receptor sites, and its flavonoid constituents, apigenin among them, have been researched for binding activity at central receptors involved in arousal. This receptor-level work is part of why passionflower is studied as a modulator of GABAergic tone rather than a blunt sedative. Human trials, while smaller and more preliminary, have explored passionflower preparations for measures of nervous tension and sleep quality; a randomised controlled trial reported improvements in self-reported stress and sleep-related measures against placebo, and reviews of this literature generally note a favourable tolerability profile. As with the adaptogen research, the biomarker and mechanistic work tends to be more consistent than the subjective outcomes, which is worth holding in mind when reading any single study.

Because the mechanism is about modulating GABA tone rather than sledgehammering the brain into unconsciousness, passionflower is often positioned as calm-without-sedation, a way to support the wind-down rather than force it. Herbal formulas frequently pair it with other traditional calming botanicals such as zizyphus and magnolia. Our Metagenics NeuroCalm is a passionflower-centred formula of this kind, aimed at the stress response and evening calm.

Putting it together: a considered approach

If there's a through-line here, it's that "stress" is not one problem with one lever. It's a system: HPA feedback, a daily cortisol curve, and the GABA-driven brake that lets you downshift at night. That's why the research-backed options tend to map onto different parts of the picture:

• Adaptogens (ashwagandha) are studied over weeks of daily use for their association with cortisol and a healthy stress response. Think of them as working on the system's set-point.
• Cortisol-support formulas layer an adaptogen with GABA-adjacent compounds like magnolia and L-theanine, for people who want a single daily foundation.
• GABA-pathway herbs (passionflower) are researched for the moment of winding down, the evening transition from wired to at-ease.

None of these is a shortcut, and none replaces the fundamentals that move the needle most: consistent sleep and wake times, morning light, movement, caffeine timing, and genuine recovery. Supplements are best understood as one considered input among those foundations, which is exactly how the research frames them.

If you'd like to explore the products discussed here, they live together in our Stress & Sleep collection, curated for evidence-minded people who want to understand what they're taking and why.

References

• Albalawi AA, et al. Dual impact of ashwagandha: significant cortisol reduction but no effect on perceived stress. A systematic review and meta-analysis. Nutrition and Health, 2025. doi:10.1177/02601060251363647
• Bachour, et al. Ashwagandha (Withania somnifera) for stress and anxiety: a systematic review and meta-analysis. BJPsych Open, 2025. PMC12242034
• Appel K, et al. Modulation of the γ-aminobutyric acid (GABA) system by Passiflora incarnata L. Phytotherapy Research, 2011. PMID: 21089181
• Harit MK, et al. A randomised, double-blind, placebo-controlled trial of Passiflora incarnata on stress and sleep. Cureus, 2024. PMC11026993

This article describes findings from published research for general educational purposes. It is not medical advice, and nothing here is intended to diagnose, treat, cure, or prevent any disease. If you take prescription medication or have a health condition, consult a qualified healthcare professional before adding a supplement.