[Yawning: from birth to senescence].

Yawning is one of the most under-appreciated behaviors. It is a stereotyped and often repetitive motor act, characterized by gaping of the mouth accompanied by a long inspiration, a brief acme followed by a short expiration. The vigor of the act may increase arousal. Although socially offensive to many, yawns often bring pleasure to the yawner. While influenced by several neurotransmitters, yawning is strongly affected by dopamine. Dopamine activates oxytocin production in the paraventricular nucleus of the hypothalamus, oxytocin may then activate cholinergic neurotransmission in the hippocampus and the reticular formation of the brainstem. Acethylcholine induces yawning via the muscarinic receptors of effectors. Other neurotransmitters can modulate its occurence like serotonin, neuropeptides, hypocretin and sexual hormones. The decrease of yawning in the elderly suggests an associated decrease of dopaminergic activity. Yawning and stretching have related phylogenetic old origins. Ethologists agree that most vertebrates yawn. Yawning is morphologically similar in reptiles, birds, mammals and fishes. They may be ancestral vestiges surviving throughout evolution with little variation. In the human embryo, yawning occurs as early as 12 weeks after conception and remains relatively unchanged throughout life. Across the life span, night sleep undergoes several age-related modifications. Theses changes concern sleep duration and the amount of REM and NREM sleep. We can describe, for the duration of REM sleep, a curvilinear trend with a steep descending slope in the last time of fetus life and the first year of life, a plateau level across childhood and adulthood, slowly lowering until age. A parallel curve demonstrates the similarity of the evolution of yawn's frequency and the amount of REM sleep. Thus, from ontogeny, phylogeny and this modelling approach emerges a pivotal link between yawning and REM sleep. Yawning is modified in some pathologies associated with aging.