Morningness–Eveningness and Sociosexuality from a Life History Perspective

Chapter 4: Morningness–Eveningness and Sociosexuality from a Life History Perspective. James Marvel-Coen, Coltan Scrivner & Dario Maestripieri. In: The SAGE Handbook of Personality and Individual Differences: Volume II: Origins of Personality and Individual Differences. Edited by: Virgil Zeigler-Hill & Todd K. Shackelford. May 2018. http://dx.doi.org/10.4135/9781526451200.n4

Basic Aspects of Morningness.Eveningness

Many human biological processes are regulated by circadian rhythms, sometimes referred to as "internal clocks". These circadian rhythms apply to hormone concentrations, brain activity, heart rate, and body temperature. In humans and many other animals, a "master clock" is attuned to a 24-hour cycle, and corresponds to sleep and wakefulness. The master clock in humans operates through the action of the suprachiasmatic nucleus (SCN) in the hypothalamus (Herzog et al., 1998). Although our circadian rhythms have been selected for based on a general pattern of light and dark, environmental factors can influence circadian rhythms, and rhythms can vary between people.

Morningness-eveningness -or chronotype- refers to the notion that individuals vary from one another in preferences for the timing of waking up and falling asleep, as well as for diurnal peaks in activity and performance, such that some individuals tend to be more active, both cognitively and physiologically, in the morning, whereas others tend to be more active in the evening (Randler et al., 2016).  Variation in morningness-eveningness tends to occur along a continuum, and the individuals at the two extremes of this continuum are often denoted as morning-types and evening-types, or "early birds" and "night owls". Research has shown that approximately 40% of individuals are either morning- or evening-types, with the other 60% falling into a more neutral category (Adan et al., 2012). Propensities for being a morning- or an evening-type are significantly heritable (e.g., Hur, 2007; Hur et al., 1998; Vink et al., 2001) but age, sex, and environment are important as well.

Children are typically morning-oriented but evening orientation tends to increase in both males and females throughout adolescence (Randler, 2011; Roenneberg et al., 2004).  Sex differences in morningness-eveninness also begin to appear in adolescence, with more males being represented in the evening type category than females (Randler, 2007).  However, these sex differences disappear after women reach menopause, suggesting that that they may be functionally linked to reproduction and be regulated by reproductive physiology, at least in women (Adan et al., 2012). Early experience and environment can influence variation in morningness-eveningness. For example, individuals who spend their first few months of life in a short photoperiod (i.e., autumn and winter) tend to be morning-types, whereas those who spend their first few months in a long photoperiod (i.e., spring and summer) tend to be evening-types (Mongrain et al., 2006; Natale and Di Milia, 2011). Latitude has also been shown to have a strong effect on chronotype, with people at northern latitudes having significantly later midpoints of sleep (Natale et al., 2009). This effect is moderated by residency type, however, with larger towns being less affected by latitude (Borisenkov et al., 2012).  Thus, it is probable that sunlight, and potentially artificial light as well, plays a role in the development and shaping of chronotype. However, this effect is not entirely clear, as evening-types tend to have been exposed to more sunlight post-birth, but less during life.