Yawning in morning and evening types.

Yawning occurs more frequently in the early morning and in the late evening, close to sleep onset and after the awakening, and it might be linked to sleep propensity. We aimed to study yawning and its temporal distribution in morning and evening subjects who display different sleep-wake and sleepiness rhythms. Sixteen healthy young adults (8 evening-types and 8 morning-types, matched for age and gender) have been selected and instructed to keep their habitual sleep schedules and to signal every yawning occurrence for three consecutive days. Results show that evening-types yawn more frequently than morning-types, particularly during morning hours. Yawning frequency decreases across daytime in evening-types reaching its lowest level in the early evening and increases thereafter. Instead, in morning-types, yawning frequency remains quite low during daytime and increases in the evening. Moreover, both morning and evening types show a progressive increase of yawning frequency in the hours preceding sleep onset, whereas they differ after the awakening. Evening-types show a higher yawning frequency that remains quite stable in the hours following the awakening, while morning-types display a decline in yawning frequency. Our findings show that the temporal distribution of yawning frequency differs between chronotypes, supporting the hypothesis that differences in sleep-wake rhythm affect yawning, which could represent a behavioural sign of sleep propensity.