MEDICAL MISCELLANY

Why do we yawn even after a long night’s sleep? YAWNING is not directly linked to the number of hours of sleep we’ve had, but is an evolutionary response to changes in our alertness.

That’s why we tend to yawn before or after sleep and at times of stress, says Simon Thompson, professor of clinical psychology and neuropsychology at Bournemouth University. A recent theory is that it’s linked to the stress hormone cortisol, which raises the temperature of the brain. ‘Yawning is thought to allow more oxygen into our lungs as this reduces our brain’s temperature,’ explains Dr Thompson. According to this theory, after a good night’s sleep we yawn because our brain’s temperature is still too high — we release cortisol to wake us up — so we yawn until it falls to a normal level.