Researchers studied 40 adults with ADHD (aged 18 to 55; 21 men and 19 women). Most of them, 31 of the 40, reported long-standing trouble falling asleep, known as sleep-onset insomnia. The other 9 did not. The team tracked sleep with logs and a wrist movement monitor over one week and measured melatonin in saliva over one night. They compared these adults with matched healthy people.
The body's evening melatonin rise marks when the internal clock is getting ready for sleep. In the adults with ADHD, this rise started 83 minutes later than in the healthy group. The delay came mainly from the group with insomnia. Their melatonin rise was 75 minutes later than in the ADHD adults without insomnia. This same group started their sleep 53 minutes later and woke 60 minutes later than the healthy group. Their daily rhythm was also weaker, with a lower swing between high and low points (10.8 ±11.7% lower). A later melatonin rise went along with a later sleep start (r = .65).
This was an observational study, so it cannot prove cause. The group without insomnia was small (9 people). The authors said they could not tell whether the shifted clock helps cause the delayed sleep or simply results from it.
Source: Van Veen M, Kooij J, Boonstra A, et al.. Delayed Circadian Rhythm in Adults with Attention-Deficit/Hyperactivity Disorder and Chronic Sleep-Onset Insomnia. Biological Psychiatry (2010). doi:10.1016/j.biopsych.2009.12.032
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