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Yawning, besides being a spontaneous behavior, can also be evoked by observing others yawn. However, contagious yawning does not always occur, depending possibly on several factors, such as one's propensity to spontaneously yawn and a heightened level of sleepiness. The aim of this study is to investigate in young adults whether contagious yawning frequency varies throughout the day, and if it is related to the daily time course of spontaneous yawning frequency and level of sleepiness. For the study, 22 subjects were instructed to log hourly, throughout wakefulness, the number of spontaneous yawns and sleepiness level. Subjects were required to continue this procedure for 3 consecutive days, after which they underwent five experimental sessions occurring at regular intervals throughout the same day. During each experimental session, subjects observed others yawn (stimulus condition) and smile (control condition). Our findings show that the contagious effect of yawning is always present throughout the daytime. Both contagious and spontaneous yawning peaked in the early morning and in the late evening, according to the sleepiness time course. However, the frequency of spontaneous yawns was remarkably lower than the frequency of contagious yawns around 19:00. This difference suggests that different mechanisms control spontaneous and contagious yawning.

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SUMMAR Y Yawning is related to sleep ⁄ wake transitions and time of day, probably reflecting the time course of sleepiness. As aging modifies sleep-wake and sleepiness rhythms, we suppose that yawning frequency and its time course vary as a function of age. Thirteen aged healthy subjects (77.15 ± 4.09 years) and 12 young adults (24.41 ± 3.31 years) were instructed to keep their habitual sleep schedules for three consecutive work-days, during which they were required to signal every yawning occurrence and to evaluate hourly their sleepiness level. Results showed that aged subjects yawn less frequently than young adults, particularly during morning and mid-afternoon hours. The time course of yawning was different between the two age groups: aged subjects showed earlier morning peak and evening rise compared with young adults; in addition, aged subjects showed two minor peaks in-between. Differences as a function of age in the time course of yawning were associated with differences in the time course of sleepiness. The only exception pertained to the early morning yawning peak, which was close to the awakening but it was not associated with high sleepiness in aged subjects. Our study discloses that aging modifies yawning frequency and its time course. Furthermore, as in the elderly yawning after the awakening is not associated with high sleepiness level as in young adult, we put forward that sleepiness level and the proximity of sleep ⁄ wake transitions could separately affect yawning.k e y w o r d s aging, sleep, sleepiness, yawning