Yawning too often? Don't blame lack of sleep. It's just your brain cooling down.

Contrary to popular belief that has long associated yawning with sleepiness, exhaustion, boredom and low oxygen levels in the blood, researchers from the University at Albany have found that yawning acts as a brain-cooling mechanism.

The brain burns up to a third of the calories we consume, and as a consequence generates a lot of heat. According to researchers Andrew Gallup and Gordon Gallup, the brain operates more efficiently when cool and yawning enhances its functioning by increasing blood flow and drawing in cooler air.

The researchers also suggest - again contrary to popular view - that yawning does not promote sleep but helps mitigate the need to sleep. Since yawning occurs when brain temperature rises, sending cool blood to the brain serves to maintain optimal levels of mental efficiency.

The researchers have announced their findings in the journal Evolutionary Psychology. Reacting to the study, Dr H N Malik, professor of physiology at AIIMS, said, "Very few studies have been conducted on yawning and so very little is known about it. Till today, most people thought yawning was a prelude to sleep. But this study changes the theory on its head. It's very interesting."

To research the theory that yawning evolved to cool the brain, the psychologists had students watch videotapes of people yawning and counted the number of contagious yawns. In one experiment, they found that 50% of the people who were instructed to breathe normally or through their mouths yawned while watching other people yawn, while those told to breathe through their nose did not yawn at all.

In another experiment, they found that subjects who held a cold pack to their forehead acted similarly to those who were instructed to breathe through their nose - they, too, did not yawn, while those who held a warm pack or a room temperature pack to their forehead yawned normally.

Evidence shows that blood vessels in the nasal cavity and face send cool blood to the brain, and by breathing through the nose or by cooling the forehead, the brain is cooled, eliminating the need to yawn. A yawn is a reflex of deep inhalation and exhalation.

An average yawn lasts about six seconds. The heart rate can rise as much as 30% during a yawn. Nearly 55% of people will yawn within five minutes of seeing someone else yawn. Blind people yawn more after hearing an audio tape of people yawning.

Researchers have believed that yawning marks the body's readiness to become alert. Paratroopers report yawning before they jump.