A new hypothesis is getting attention in ADHD research circles: that for a significant subgroup of people with ADHD, what's really going on is at least partly a circadian rhythm problem — a body clock running late.
It comes from a paper published in Frontiers in Psychiatry in December 2025 by Dr Brandon Luu (Queen's University) and Dr Nicholas Fabiano (University of Ottawa), titled “ADHD as a circadian rhythm disorder: evidence and implications for chronotherapy.” It's worth being precise about what it actually claims, because the honest version is more interesting — and more useful — than the headline.
What the paper actually says
The authors are careful not to claim ADHD simply is a circadian disorder. Their argument is narrower: that circadian rhythm dysfunction is a “clinically significant and highly prevalent phenotype” in a substantial subgroup of people with ADHD — not everyone, but not a small minority either.
The evidence they cite
- ✓Sleep disturbance is common: insomnia and sleep problems affect up to 80% of adults and 82% of children with ADHD, and delayed sleep-wake timing shows up in as many as 78%.
- ✓Melatonin timing is delayed: dim-light melatonin onset (DLMO), the biological marker of when your body actually starts winding down for sleep, is delayed by roughly 45 minutes in children and around 90 minutes in adults with ADHD compared to people without it.
- ✓Cortisol rhythms are disturbed too: adults with ADHD show disrupted rhythmicity in both melatonin and cortisol, and children with ADHD tend to have lower morning cortisol specifically.
- ✓Clock genes look different: studies of BMAL1 and PER2 — genes that help run the body's internal clock — found weaker, less rhythmic expression in people with ADHD, and symptom severity tracked with how disrupted that rhythm was.
What “chronotherapy” means in practice
The paper reviews a handful of small trials testing whether shifting someone's body clock earlier — through timed light exposure, low-dose melatonin, or both — changes anything. A few findings stood out:
- ✓Low-dose melatonin (3–6mg, taken in the evening) shifted DLMO earlier by around 44 minutes in one trial of 101 medication-free children over 4 weeks.
- ✓Morning bright light therapy (10,000 lux) shifted DLMO earlier by about 31 minutes on its own.
- ✓Combining both shifted the body clock the most — roughly 2 hours earlier in adults — and that amount of shift was the strongest predictor of any improvement in ADHD symptom scores.
- ✓A simpler, drug-free approach also showed promise: fixed wake times, more morning light, less evening light, no late dinners, no caffeine after 3pm, and morning exercise.
The important caveat, in the authors' own words: “evidence on remission of ADHD with circadian interventions is lacking.” Shifting someone's body clock earlier is not the same as proving it improves their ADHD. One of the studies cited found the biological clock shift happened within 4 weeks, but with “no identified changes in cognitive performance or problem behavior” in that timeframe. The authors are explicit that this should be researched as an adjunct to existing treatment, not a replacement for it, and that better-designed trials are still needed before anyone can say circadian correction reliably helps ADHD symptoms.
There's also a real chicken-and-egg question the authors flag themselves: does a delayed body clock contribute to ADHD symptoms, or does living with ADHD (later bedtimes, more evening screen use, less structured mornings) delay the body clock instead? The paper doesn't resolve this, and says so plainly.
Why this is worth knowing about
Even without a proven treatment effect yet, the sleep and circadian evidence alone is useful. If you have ADHD and also struggle with a genuinely delayed sleep schedule — not just “bad habits,” but a body clock that seems to run hours later than everyone else's — that's now something with real research behind it, not just something to feel guilty about. It's also a reasonable thing to mention at an assessment: sleep and circadian patterns are relevant clinical information, not a side note.
What it isn't, yet, is a reason to self-prescribe melatonin or buy a light box expecting it to treat ADHD on its own. The doses and timing used in these trials were specific and supervised — worth discussing with a clinician rather than guessing at.
If sleep is part of what's making your day-to-day harder, that's exactly the kind of thing a proper assessment can help untangle. Our free 3-minute screener is a starting point, and our directory lists accredited private ADHD clinics across the UK.
Source: Luu B, Fabiano N. ADHD as a circadian rhythm disorder: evidence and implications for chronotherapy. Front Psychiatry. 2025 Dec 10. doi:10.3389/fpsyt.2025.1697900. Open access.
This article is for informational purposes only and does not constitute medical advice. The research described is preliminary and does not establish that circadian interventions treat or remit ADHD. Do not start melatonin or light therapy for ADHD without speaking to a clinician. ADHD Navigator is not a medical provider and does not offer diagnostic or treatment services.