ADHD is the most common neurodevelopmental disorder (Faraone et al., 2003) and affects about 3–6% of children (Tannock 1998). ADHD is defined by either an attentional dysfunction, hyperactive/impulsive behaviour or both (DSM-IV; American Psychiatric Association, 1994). Therefore, the diagnosis of ADHD has three subtypes: the Inattentive subtype (ADHD/IA), which is characterised by significant levels of inattention but subthreshold levels of hyperactive/ impulsive symptoms, the Hyperactive/Impulsive subtype (ADHD/HI), which is defined by hyperactivity/ impulsivity but not of inattention symptoms, and the Combined Inattentive-Hyperactive/Impulsive subtype (ADHD/C), which is characterised by maladaptive levels of both symptom clusters.
Morningness is a stable characteristic which reflects the phase of circadian system. It is a continuum with evening types at one end and morning types on the other. Previous studies have found that the evening orientation might be a risk factor for various disorders including depression and personality disorders. Morningness is also a heritable trait (Vink, Groot, Kerkhof, & Boomsma, 2001) and determined by genetic factors (Mishima, Tozawa, Satoh, Saitoh, & Mishima, 2005). Impulsivity and novelty seeking, two characteristics associated with particular ADHD subtypes are negatively related to morningness. Specifically, evening oriented individuals often score higher on tests assessing those traits. In addition to that, there is evidence that morningness is implicated in the variation of performance (Natale, Alzani, & Cicogna, 2003). Variability in various cognitive tasks is a common finding in many studies examining individuals with ADHD. Individuals with ADHD have also been found to experience a number of sleep related disorders such as sleep-onset difficulties, agitated sleep, and a higher number of nocturnal awakings.
Caci et al. examined the relationship between morningness and ADHD. Their hypothesis was that adults suspected of having ADHD are more evening oriented than are adults without ADHD. They recruited 354 participants and assessed their scores in the Composite Scale of Morningness (CSM), a measure of morningness, and the Adult Self-Report Scale v1.1 (ASRS), a self-reported questionnaire used for screening of ADHD in adults. ASRS includes two subscales for inattention and hyperactivity symptoms. This allowed Caci et al to examine the relationship between possible ADHD subtypes and morningness.
The results of the study confirmed the hypothesis; participants with higher scores on the ASRS reported having an evening orientation. The effect was stronger in participants with higher scores on the subscale of inattention. No correlation was found between hyperactivity and morningness. This provides evidence for the existence of different endophenotypes in ADHD. Since the sample used in this study consisted of healthy volunteers, it would be interesting to try to replicate this finding in diagnosed individuals with ADHD.