What’s the Relationship Between Daytime Sleepiness and Cognitive Functioning in Adults with ADHD?

Sleep disorders are one of the most commonly self-reported comorbidities of adults with ADHD, affecting 50 to 70 percent of them. A team of British researchers set out to see whether this association could be further confirmed with objective sleep measures, using cognitive function tests and electroencephalography (EEG).

Measured as theta/beta ratio, EEG slowing is a widely used indicator in ADHD research. While it occurs normally in non-ADHD adults at the conclusion of a day, during the day it signals excessive sleepiness, whether from obstructive sleep apnea or from neurodegenerative and neurodevelopmental disorders. Coffee reverses EEG slowing, as do ADHD stimulant medications.

Study participants were either on stable treatment with ADHD medication (stimulant or non-stimulant medication), or on no medication. Participants had to refrain from taking any stimulant medications for at least 48 hours prior to taking the tests. Persons with IQ below 80 or with recurrent depression or undergoing a depressive episode were excluded.

The team administered a cognitive function test, The Sustained Attention to Response Task (SART). Observers rated on-task sleepiness using videos from the cognitive testing sessions. They wired participants for EEG monitoring.

Observer-rated sleepiness was found to be moderately higher in the ADHD group than in controls. Although sleep quality was slightly lower in the sleepy group than in the ADHD group, and symptom severity slightly greater in the ADHD group than the sleepy group, neither difference was statistically significant, indicating extensive overlap.

Omission errors in the SART were strongly correlated with sleepiness level, and the strength of this correlation was independent of ADHD symptom severity. EEG slowing in all regions of the brain was more than 50 percent higher in the ADHD group than in the control group and was highest in the frontal cortex.

Treating the sleepy group as a third group, EEG slowing was highest for the ADHD group, followed closely by the sleepy group, and more distantly by the neurotypical group. The gaps between the ADHD and sleepy groups on the one hand, and the neurotypical group on the other, were both large and statistically significant, whereas the gap between the ADHD and sleepy groups was not. EEG slowing was both a significant predictor of ADHD and of ADHD symptom severity.

The authors concluded, “These findings indicate that the cognitive performance deficits routinely attributed to ADHD … are largely due to on-task sleepiness and not exclusively due to ADHD symptom severity. … we would like to propose a simple working hypothesis that daytime sleepiness plays a major role in cognitive functioning of adults with ADHD. … As adults with ADHD are more severely sleep-deprived compared to neurotypical control subjects and are more vulnerable to sleep deprivation, in various neurocognitive tasks they should manifest larger sleepiness-related reductions in cognitive performance. … One clear testable prediction of the working hypothesis would be that carefully controlling for sleepiness, time of day and/or individual circadian rhythms would result in substantial reduction in the neurocognitive deficits in replications of classic ADHD studies.”

Bartosz Helfer, Natali Bozhilova, Ruth E. Cooper, Joanna Ismene Douzenis, Stefanos Maltezos, Philip Asherson, “The Key Role of Daytime Sleepiness in Cognitive Functioning of Adults with ADHD,” European Psychiatry (2020), https://doi.org/10.1192/j.eurpsy.2020.28.

ADHD and Insomnia

ADHD itself is associated with sleep difficulties, independent of ADHD medications. Thus, it is very important that sleep quality is assessed prior to treatment so that the changes due to treatment can be correctly inferred.

(Editor’s Note: See our Ask the ADHD Experts session on ADHD and Sleep.)

In clinical trials of stimulant ADHD medications, insomnia is typically noted a side effect of the medications. But most of these studies have used subjective patient or parent reports of sleep quality. A new meta analysis, reviews 9 studies of a total of 246 patients enrolled in randomized controlled trials of a stimulant medication.

Ask_the_ADHD_Experts_-_Prescribing_MedicationsTo be included, studies must have had an objective measure of sleep quality, either polysomnography or actigraphy. The analysis showed that stimulant medications led to a) a longer time to get to sleep; b) worse sleep efficiency and c) a shorter duration of sleep. Some of these sleep measures worsened with an increasing number of doses and a shorter time on medication.

Given the adverse effects that lack of sleep can have on cognition and behavior, these data provide further impetus for clinicians, parents and patients to monitor the effects of stimulant ADHD medication on sleep and to take appropriate action (e.g., dose reduction, change of medication) as warranted.


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