Ashwagandha is an adaptogenic herb that primarily reduces stress hormones, improves sleep, and supports physical performance. Clinical trials show it lowers cortisol (your body’s main stress hormone) by 11% to 33% depending on the dose, making it one of the more well-studied herbal supplements for stress management. Its effects extend beyond stress relief into sleep quality, cognitive function, testosterone levels, and exercise recovery.
How It Lowers Stress and Cortisol
Ashwagandha’s most researched effect is its ability to reduce cortisol, the hormone your body releases when you’re under physical or psychological stress. Across multiple clinical trials in stressed but otherwise healthy adults, cortisol levels dropped anywhere from 11% to 33% compared to baseline. The reduction appears to be dose-dependent: one study found 125 mg twice daily lowered cortisol by 16.5%, while 300 mg twice daily brought it down by 32.6%.
The active compounds, called withanolides, appear to work through at least two pathways. First, they bind to the same receptors in the brain that cortisol uses, which helps dial down the stress response at the hormonal level. Second, they influence GABA signaling, the same calming brain chemistry that anti-anxiety medications target. This GABA activity likely explains why ashwagandha helps with both stress and sleep, since poor sleep drives cortisol higher during the day, and high cortisol makes sleep worse. Breaking that cycle is a big part of how the herb works.
Sleep Quality Improvements
A meta-analysis published in PLOS ONE found that ashwagandha meaningfully improved two key sleep measures: how long it takes to fall asleep (sleep onset latency) and the percentage of time in bed actually spent sleeping (sleep efficiency). The effects were statistically significant across multiple trials involving nearly 300 participants. Most sleep studies used doses of 300 mg taken twice daily over six to eight weeks.
The sleep benefits seem strongest in people who already have poor sleep. If you fall asleep easily and sleep through the night, you’re less likely to notice a difference. But for people dealing with stress-related insomnia or unrefreshing sleep, the combination of lower cortisol and enhanced GABA activity can produce a noticeable improvement in how quickly you fall asleep and how rested you feel in the morning.
Physical Performance and Muscle Recovery
Ashwagandha has shown benefits for both strength training and cardiovascular fitness. A double-blind, placebo-controlled trial in healthy athletic adults found statistically significant improvements in VO2 max, a measure of how efficiently your body uses oxygen during exercise. Participants taking ashwagandha also showed better recovery from exercise-induced muscle damage.
For resistance training specifically, research has found increased muscle mass in people supplementing with ashwagandha alongside a strength program. The herb appears to support neuromuscular coordination and lower limb muscle strength, which makes it popular among athletes and regular gym-goers. These effects likely stem from the cortisol reduction (since chronically elevated cortisol breaks down muscle tissue) combined with the testosterone increase described below.
Testosterone and Male Reproductive Health
In men, ashwagandha has a notable effect on testosterone. One trial found an 18% increase in testosterone in the supplementation group, while the placebo group actually saw an 11% decline. Earlier research also found improvements in sperm quality in men dealing with stress-related fertility issues, with cortisol reductions of 28% to 32% in those populations. The testosterone boost appears connected to the overall stress reduction rather than a direct hormonal mechanism, meaning it’s most pronounced in men whose testosterone is suppressed by chronic stress or poor sleep.
Cognitive Function and Focus
Ashwagandha improves several markers of cognitive performance, particularly reaction time, memory recall, and sustained attention. In a controlled trial published in Nutrients, participants taking ashwagandha showed improvements in word recall (both speed and accuracy), picture recognition, and the ability to correctly identify targets in attention tasks. After 30 days of supplementation, the ashwagandha group maintained or improved their cognitive scores while the placebo group showed measurable declines in several areas.
One of the more interesting findings was in the Stroop test, which measures your ability to process conflicting information. The ashwagandha group maintained their accuracy and improved reaction times, while the placebo group’s accuracy dropped. These cognitive benefits appear both acutely (on the first day of supplementation) and after sustained use, though the 30-day results were more consistent.
Typical Dosage and Extract Types
The two most common standardized extracts you’ll find in supplements are KSM-66 and Shoden, and they differ significantly in concentration. KSM-66 is a root-only extract standardized to more than 5% withanolides, typically taken as 600 mg daily (two 300 mg capsules). Shoden is a root and leaf extract with a much higher withanolide concentration, taken at just 120 mg daily (two 60 mg capsules containing 21 mg of withanolide glycosides each).
Most clinical trials showing benefits for stress, sleep, and cortisol used doses in the range of 300 to 600 mg daily of a root extract, taken for six to twelve weeks. The effects aren’t immediate for most outcomes. Cortisol and sleep improvements typically become noticeable within two to four weeks, while muscle and testosterone changes take closer to eight weeks.
Thyroid Effects
Ashwagandha can increase your levels of thyroid hormones T3 and T4, likely as a downstream effect of regulating the stress response. For people with normal or underactive thyroid function, this is generally not a problem and may even be mildly beneficial. But if you have an overactive thyroid (hyperthyroidism), ashwagandha can push thyroid hormones higher and worsen symptoms. Case reports have documented thyroid dysfunction in individuals who weren’t previously diagnosed with thyroid problems, though this is rare.
If you take thyroid medication such as levothyroxine, combining it with ashwagandha can interfere with your dosage balance. Your medication is calibrated to achieve specific hormone levels, and ashwagandha’s own thyroid-stimulating effect can throw that off.
Safety and Side Effects
At standard doses, ashwagandha is well tolerated by most people. The most common side effects are mild: drowsiness, stomach upset, and occasionally loose stools. A large-scale safety analysis identified only nine publications reporting potential liver toxicity, and most of those cases involved people with pre-existing liver conditions, very high doses, or simultaneous use of other supplements. In clinical monitoring studies, the few participants who showed elevated liver enzymes saw them return to normal after stopping supplementation.
Drug Interactions to Know About
Because ashwagandha has calming and sedative properties, combining it with other sedating substances can cause excessive drowsiness or slowed breathing. This includes benzodiazepines, opioid pain medications, prescription sleep aids, melatonin, and kava.
- Thyroid medications: Ashwagandha’s ability to raise T3 and T4 can conflict with drugs designed to regulate those same hormones.
- Diabetes medications: Ashwagandha may lower blood sugar on its own, and combining it with diabetes drugs increases the risk of blood sugar dropping too low.
- Blood pressure medications: Similar stacking risk, where ashwagandha’s mild blood-pressure-lowering effect could amplify the medication and cause dangerously low readings.
People who are pregnant, have autoimmune conditions, or are scheduled for surgery (due to its effects on blood sugar and blood pressure) are also typically advised to avoid it.