The hypothalamic peptides ACTH and MSH, given intracranially, induced a peculiar stretching and yawning syndrome associated with penile erection and ejaculation (Donovan, 1978).

Experiments on rats have shown that increased serotoninergic activity induces yawning (Holmgren, Urba-Holmgren, & Rodriguez, 1980). Bombardment with hypnagogic derivatives of the pontine serotoninergic raphe nuclei induces yawning before the onset of sleep. Serotonin is considered to be a positive modulator of the cholinergic activity that sets off the yawning reflex (Holmgren et al.). Serotoninergic-induced. yawning increases cholinergic activity, and through an inhibitory feedback mechanism, decreases serotoninergic activity. As long as yawning continues, sleep initiation is avoided (Friedell, 1974; Lehmann, 1979; Twiest, 1974). The end of yawning is associated with a feeling of comfort, the result of a temporary victory over the onset of sleep.

yawning leads to:
- +O2, -CO2 (reversal of blood hypoxia)
- increase of metabolic rate
- inhalation stretches the bronchial musculature, stimulating the vagal terminals and the cholinergic pathway, which dilate arterioles, thus lowering periperhal circulation resistance and enhancing blood flow (Friedell, 1974; Lehmann, 1979; Twiest, 1974).
- gaping jaws & stretched limbs have important circulatory effects
- lateral pterygoids and the soleus muscle contain rich venous plexi within their mass, which empty with the contraction and are therefore called "peripheral hearts". Through this mechanism, venous return is enhanced (Bhangoo, 1974; Last, 1963; Siegal, 1974).
- diaphragm contraction (associated with inspirium/inhalation) enhances venous return
- hyperoxygenation and increased blood circulation stimulate all the cephalic structures, including the AARS (Montagu, 1962).

boredom leads to:
- hypoxia
- reduced metabolic rate
- increased secretion of hypnogenic-derivatives

AARS reverses drowsiness:

reticular neuronal yawning complex is interconnected with:
- three aggregates of respiratory neurons located in the medulla, dorsal, ventral and parabrachialis
- the motor nuclei of the fifth, seventh, ninth, tenth and twelfth cranial nerves
- the phrenic nerve
- the motor roots of the intercostal nerves
- the parasympathetic terminals of the lacrimal gland

yawning physically occurs by:
- opening of the mouth by the digastric and milohyoid muscles
- retraction of the jaw achieved by the posterior fibers of the temporal muscles and the geniohyoid muscles

induces yawning:
- serotonin
- bombardment with hypnagogic derivatives of the pontine serotoninergic raphe nuclei

serotoninergic-induced yawning:
- increases cholinergic activity
- decreases serotoninergic activity (via inhibitory feedback mechanism)


Yawning associated with orgasm appeared as a side effect of clomiprarnine treatment in a depressed patient and disappeared when treatment ceased (McLean, Forsythe, & Kapkin, 1983).