Objective Why do we yawn

1
Spontaneous yawning does not induce an arousal in
humans Adrian G. Guggisberg, Johannes Mathis,
Uli S. Herrmann, Christian W. HessCenter for
Sleep Medicine, Department of Neurology,
University Hospital, Inselspital, 3010 Berne,
Switzerland

• Methods
• Maintenance of Wakefulness Tests (MWT) of 16
  patients complaining of excessive daytime
  sleepiness (4 females, mean age 37.4 years, age
  range 18 62 years) having yawned at least 4
  times during the test were selected.
• Two events (conditions) were analyzed
• Yawns with accompanying movements
• Isolated movements without yawning (control)
• EEG spectra and heart rate variability (HRV)
  changes associated with yawning were compared to
  changes associated with voluntary body movements
• Artifact-removal procedures for EEG-signals
• The EEG-time-frames corresponding to the actual
  yawning- or movement-events were removed, since
  they were often heavily contaminated by
  EMG-activity. Instead, segments before and after
  yawning or movements were analyzed.
• EEG components corresponding to eye movements,
  ECG-activity or single electrode-artifacts were
  removed by means of an independent component
  analysis (ICA).
• The remaining segments were visually inspected
  and rejected if not artifact-free.

• Objective Why do we yawn?
• Previous work has shown that yawning is
  independent of blood gases (O2 and CO2)
• Current hypotheses of the function of yawning
• It increases the arousal level
• It is a form of non-verbal communication among
  members of a social group, signaling drowsiness,
  boredom, and/or psychological stress
• This study aimed at testing empirically the
  arousal hypothesis

• Results
• Yawns were preceded and followed by a
  significantly greater delta-activity in EEG than
  movements (Fig. 1 and 2, , plt0.008 ).
• Alpha-rhythms were attenuated and decelerated
  after yawning (Fig. 1 and 2, , p0.01 ),
  whereas they were attenuated and accelerated by
  movements ( , plt0.02 ).
• A significant transient increase of HRV occurred
  at the onset of yawning and movements (Fig. 3,
  , plt0.0001 ), which was followed by a
  significant slow decrease peaking 17 s after
  onset ( , plt0.0001 ). No difference in HRV
  changes was found between yawns and movements.

• Conclusions
• The increased delta activity before and after
  yawning indicates that it occurs during periods
  with increased drowsiness and sleep pressure
• But yawning does not accelerate the alpha rhythm
  (as isolated movements do) or reduce theta/delta
  activity, and therefore does not increase the
  arousal level of the brain.
• The autonomic activation after yawning occurred
  to the same extent after isolated movements, and
  is therefore unspecific.
• Therefore, the arousal hypothesis of yawning must
  be rejected.

Fig. 1
Fig. 3
Fig. 2