Drivers instinctively open the car window if they feel drowsy, but, until now, scientists did not know why keeping a cool head should keep people alert.
The latest 'theory of alertness' comes from a team at Harvard University, which describes its work in a paper published in today's issue of Science magazine.
The team, led by Professor Robert Greene of Harvard Medical School's department of psychiatry, examined a group of neurons in the brain known to control arousal and a key 'messenger' compound, called adenosine, with a role in storing energy in the body's cells. They found that adenosine also has a critical part to play in communication between one neuron and another, blocking the passage of messages. The more adenosine there is, the less active the brain will be.
'When your cells are tired, adenosine levels go up, firing slows down and the more drowsy and fatigued you feel,' he said. 'After prolonged exercise you will often feel drowsy, because the more exercise you do the more your brain heats up, and up go your adenosine levels.'
Prolonged mental activity could have the same effect, he said. Thinking hard for a long time could heat up the brain enough to induce sleepiness.