According to a new study, definite eveningness is associated with self-perceived loneliness. Further, lonely evening-type individuals have smaller right hippocampal volume. This research was published in the journal Biological Rhythm Research.
Eveningness refers to the preference to wake up at a later hour, retire at a later hour, and plan activities at later hours in the day. This preference has been associated with poorer health outcomes, such as type 2 diabetes, cardiovascular disease, and increased mortality. It has also been linked to psychiatric disorders such as depression, and personality traits such as neuroticism. Eveningness has further been associated with negative biases in emotional processing and impaired emotion regulation. Contrasting this, morningness has been associated with increased conscientiousness, openness and agreeability.
One theory explaining the association between eveningness and poor physical and mental health outcomes is related to loneliness or social isolation, which reflects a disparity between a person’s preferred and actual social relations. This differs from solitude, which refers to the choice to be socially isolated, perhaps for personal growth or to take a step back from societal demands. Social connectedness is positively associated with health and well-being, while a lack of it has been linked to physical and psychiatric disorders (e.g., depression). Various magnetic resonance imaging (MRI) studies have explored the association between grey matter volume and loneliness, finding conflicting results.
In the current work, Ray Norbury sought to 1) determine whether perceived loneliness varies between morning and evening-type individuals, and 2) to determine the impact of perceived loneliness on subcortical volume, specifically, the amygdala and hippocampus.
A total of 4684 adults between the ages 40-70 enrolled in the UK Biobank Resource were involved in this research. Diurnal preference was assessed by asking: “Do you consider yourself to be definitely a morning person/more a morning than an evening person/more an evening than a morning person/definitely an evening person?” Information pertaining to age, sex, sleep duration (including naps), and socioeconomic status were also gathered. Mental health was determined by asking participants whether they have previously been diagnosed with mental health problems by a professional (i.e., a doctor, nurse, psychologist, or therapist). Perceived loneliness was assessed by a single question: “Do you often feel lonely?”
T1-weighted anatomical images were gathered using a Siemens Skyra 3 T scanner, fitted with a 32-channel head coil in accordance with previously established procedures. Processing included subcortical segmentation, yielding volumes for the left and right hippocampus and amygdala. Further, whole brain images were segmented into three tissue types, including grey and white matter, as well as cerebrospinal fluid, and summed, to yield an estimation of intracranial volume.
Norbury found that morning-type individuals were significantly older and less deprived than evening-types, though there were no sex differences, or differences in sleep duration among the two groups. Diurnal preference was a predictor of loneliness, and this effect remained when adjusting for demographic markers. Further, women were more likely to endorse loneliness.
The researcher also observed that definite-morning lonely participants had larger hippocampal volume compared to definite-evening lonely participants. Definite-evening non-lonely participants had greater right hippocampal volume compared to definite-evening lonely participants, and definite-morning non-lonely individuals had greater volume than definite-evening lonely participants.
The data support that eveningness is associated with greater perceived loneliness, and provide partial support for the second hypothesis, revealing smaller volume in the right but not left hippocampus, nor the amygdala.
The author notes potential limitations. First, a single question was used to determine perceived loneliness, simplifying a construct that is composed of a multitude of dimensions relating to the intimate/emotional, relational/social, and collective. Thus, this work cannot address specific components of loneliness as they relate to diurnal preference.
As well, given the cross-sectional nature of this work, temporal incidence cannot be inferred from the data. It could be that social connectedness in evening-types declines in late adolescence, as young adults transition to a more morning profile. The individuals that retain a more evening-type preference gradually lose touch with social networks and group membership, as their peers adopt a schedule that is more in tune with changing circadian typology, thus isolating evening-prone individuals. However, the researcher notes this is purely speculative and requires further investigation.
The study, “Night Owls and Lone Wolves”, was authored by Ray Norbury.
