There may be bigger differences between early birds and night owls than the time they go to sleep.
In a new study conducted by Dario Maestripieri of the University of Chicago, bedtime might also be linked to relationship orientation.
As research attests, there is considerable variation in our individual preferences to go to sleep early or late in the evening, or to wake up early or late in the morning. At one end of the spectrum, early birds tend to retire and rise early; they are cognitively sharpest in the morning and don't perform as well in the evening. Conversely, night owls both go to bed and wake up late; they are cognitively sharpest in the evening and don't perform as well the morning. While many of us are firmly in either the early bird or the night owl camps, others fall somewhere in between or do not have an identifiable sleep pattern.
These individual differences in morningness vs. eveningness—technically known as chronotypes—are stable over time and, to some degree, genetic, although environmental factors, such as geography, season, and work schedules, can also influence our sleep patterns.
Studies to date have consistently found that men and women differ in their sleep patterns—though not their amount of sleep. On average, men sleep less than women, across all ages. With the onset of puberty, however, they begin to demonstrate an increased night-owl tendency, as compared with women. This gender difference in sleep patterns extends into adulthood but declines or disappears after women reach menopause. These differences have physiological manifestations: Melatonin levels in women peak earlier at night than in men, and in both men and women, skin conductance peaks earlier in the day for early birds than for night owls.
How did these differences in sleep patterns arise in the first place? Davide Piffer developed a “hypothetical scenario” for the evolution of the night-owl pattern. He proposed that since humans evolved from primate ancestors who were active during the daytime (diurnal), morningness was probably the ancestral evolutionary condition, or baseline, for our species. So eveningness may actually be a relatively new trait that potentially evolved because it offers mating advantages. As predatory and ecological threats decreased during early human evolution, the late evening hours may have become an optimal time to socialize and mate because that's when adults were freed up from hunting and gathering, as well as child care.
Both men and women may have benefited from these new social and mating opportunities, especially those who were single or cuckolding their partners—that is, those with short-term mating orientations. In this scenario, night-owl men would have had higher reproductive success than early birds, so a genetic predisposition for eveningness gradually spread in the male population. (Early-bird males weren't shut out from the mating game in this scenario—they would have enjoyed reproductive success through relationships with faithful early-bird women).
Still, since eveningness boosted the fitness of men more than that of women, eveningness became more prevalent in males. In studies based on this hypothesis, Piffer and his colleagues found that eveningness in men was associated with a higher number of sexual partners. Eveningness was associated with extraversion and novelty-seeking, traits that are also associated with a short-term mating orientation.
The relationship between chronotype and mating orientation, however, remains unclear. This is especially true in determining whether there is an association between eveningness and short-term mating for women, a question that has received little study. Maestripieri set out to test this relationship. Expanding on the work of Piffer and his collaborators, he reasons that both men and women can and do elect short-term mating and the night-owl sleep pattern. Specifically, he hypothesized that night-owl women should be more similar to night-owl men than to early-bird women, and should show a comparatively greater tendency towards a short-term mating orientation.
For his study, Maestripieri recruited master’s students at the University of Chicago's Booth Business School. The participants completed self-report questionnaires about their sleep patterns, sleep amount, relationship status, and number of previous sexual partners.
Maestripieri's hypothesis that eveningness should be associated with a greater tendency for a short-term-relationship orientation was supported in his results from both males and females: Night-owl men and women were more likely than early-bird men and women to be single than in a relationship. Further, night-owl males had more sexual partners than early-bird males (11.00 vs 6.36).
Maestripieri points out that this difference was not statistically significant but might have been if the sample size had been larger. He also cautions that this study had other methodological limitations, including participants who were MBA students with particular demographic and sleep-pattern profiles. Nonetheless, he asserts, his study can help inform future investigations of this curious human dynamic.
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