The two conditions look different in the research. Across 34 studies, autistic people had lower resting vagal heart rate variability (HRV) than non-autistic people, a medium-sized difference (Hedges’ g = -0.52), and their HRV changed less in response to social stress (Cheng and colleagues, 2020). For ADHD, a meta-analysis of eight resting studies found no difference from controls at all (Koenig and colleagues, 2017). Neither finding makes HRV a test for either condition.
Autism
The 2020 meta-analysis pooled 34 studies that compared HRV in autistic and non-autistic people:
- At rest. Parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system measures were lower in the autistic groups, with g = -0.52. Respiratory sinus arrhythmia Respiratory sinus arrhythmia The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV. More about Respiratory sinus arrhythmia , the breathing-linked swing in heart rate, was lower too (g = -0.59).
- Under social stress. When the task was social, HRV reactivity was lower in autistic participants than in others (g = -0.46), and the same held during debriefing afterwards (g = -0.50). For respiratory sinus arrhythmia, the gap was largest under social stress (g = -0.72).
The authors called low HRV, and especially a muted response to social stress, a potential biomarker for autism. That is a research label. The effect sizes mean the groups overlap heavily: many autistic people have ordinary HRV, and many non-autistic people have low HRV for other reasons.
ADHD
Here the picture depends on how HRV was measured:
- Resting. Eight studies with 587 participants found no difference in short-term vagal HRV between people with ADHD and controls (g = 0.06), in adults and children alike. The authors concluded that, unlike anxiety and depression, ADHD is not linked with altered resting vagal tone Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone (Koenig and colleagues, 2017).
- During tasks. Thirteen studies with 869 people with ADHD found slightly lower vagal HRV during or after a task demand (g = 0.20), with high heterogeneity and some sign of small-study bias (Robe and colleagues, 2019).
So a morning or overnight reading, which is a resting measure, is not expected to differ with ADHD. Any difference shows up in how the heart responds to a demand, which a wearable does not measure.
Stimulant medicines
Stimulants raise heart rate. A review of 10 trials in adults found resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate about 6 beats a minute higher on a stimulant, with a small rise in blood pressure (Mick and colleagues, 2013). A faster heart leaves less room for beat-to-beat variation, so some fall in HRV is plausible. For children and teenagers, a 2026 review of methylphenidate found no clear change in resting HRV, and rated the evidence as very low certainty (Gianini and colleagues, 2026). See do medications affect HRV?
How to read your own data
- Do not read a diagnosis into a number. HRV cannot screen for autism or ADHD. The group differences are averages, and sleep, alcohol, illness and age move the number more.
- Tag medicine changes. If you or your child start, stop or change a stimulant, note the date and compare from there. Expect a higher resting heart rate on treatment.
- Use your own range. Compare the 7-day average with your usual range, as the lesson on building your HRV baseline shows.
- Look at sleep. Both conditions often come with disturbed sleep, which lowers HRV on its own. Read HRV and sleep.
Anxiety and depression, which often travel with both conditions, have their own pages: does anxiety lower HRV? and does depression lower HRV?
When to talk to a doctor
Never start, stop or change a medicine because of a wearable reading. Talk to the prescriber if a stimulant comes with palpitations, chest pain, fainting or a resting heart rate that stays well above its usual level. HRV cannot diagnose anything, and this page is not medical advice.