The thermoregulatory theory posits that yawns function to cool the brain in part due to counter-current heat exchange with the deep inhalation of ambient air. In support of this theory, previous cross-cultural research on humans has shown that self-reported contagious yawning frequency varies between seasons with distinct ambient temperature ranges. However, it remains possible that differences in yawning across seasons are a result of physiological circadian changes across the year rather than variation in ambient temperature. In an attempt to address this question, here we discuss the results of a study investigating the variation in the frequency of self-reported contagious yawning within a restricted range of a single season in one geographic location. A total of 142 pedestrians were recruited outdoors during an 18-day period over the summer in an equatorial monsoon climate in southern Florida, USA. Consistent with the thermoregulatory theory of yawning, results showed that self-reported contagious yawning frequency varied predictably across temperature gradients. This was true after statistically controlling for relative humidity, time of day, time spent outside, testing day, age of participant, and amount of sleep the night before. These findings provide further evidence suggesting a brain cooling function to yawning.

A growing number of studies on non-human animals have documented that stressors modulate the expression of yawning. In particular, recent experimental research shows that yawns are initially inhibited following physical stress, but then become potentiated thereafter. However, stress-induced yawning in humans has yet to be demonstrated experimentally. Here, we investigated the temporal relationship between self-reported contagious yawning and an acute physical stressor in 141 human subjects in the laboratory. Using a 2 × 2 between-subjects design, participants either underwent the cold pressor test (CPT) or a matched control condition prior to viewing a contagious yawning stimulus that was either displayed immediately thereafter or following a 20-min delay. Consistent with the comparative literature, we show an interaction between stress and time conditions, whereby both the incidence and frequency of yawning are lowest in the immediate-CPT trials and highest in the delayed-CPT trials. These findings support a homologous effect of acute physical stress on yawning across birds and mammals that may be related to an adaptive thermoregulatory and arousal function.