The Empty Pillow Effect: Proposed as an evolutionary psychology construct of nocturnal attachment desire

Abstract Nighttime emotional vulnerability is a widely observed yet not yet fully explained phenomenon. Existing studies have offered partial explanations from various perspectives—such as emotional circadian rhythms, the evolutionary theory of loneliness, and attachment theory—but none have addressed a core question: Why do negative emotions experienced during nighttime solitude often feature "a craving for companionship" as their central characteristic, and why does this craving persist even when individuals are socially engaged during the day? This paper proposes the Nocturnal Affiliation Craving (NAC)—a nocturnal affiliation desire—and defines it as an evolutionarily shaped nocturnal safety-monitoring mechanism: it detects whether an individual perceives a safety signal at night; when such a safety signal is absent, it is automatically activated, giving rise to a complex state of discomfort and driving the individual to unconsciously seek social proximity in order to restore the safety signal. The activation of NAC is regulated by the individual's physiological resources and may be suppressed under conditions of extreme fatigue. This paper systematically elaborates on the evolutionary origins of NAC, its triggering conditions, its observable characteristics, and its fundamental distinction from general loneliness, attachment anxiety, and emotional circadian rhythms; it also derives nine testable hypotheses to guide future empirical research.