Scientists have discovered that lack of sleep triggers off signals in the brain that make us crave high-calorie food, especially junk food.

According to two new studies presented yesterday at the Associated Professional Sleep Societies' annual conference in the USA, lack of sleep stimulates areas of the brain that seek pleasure. Investigators suggest that feelings of hunger and dietary habits related to sleep are generated in part by intestinal glands related to appetite.

In one of the studies, carried out at the University of California in Berkeley, 23 young adults were asked to grade various plates of food whilst an MRI scan was carried out on their brains. Some of them had been awake non-stop for the previous 24 hours, whereas others were well rested. Those volunteers who had not slept for a whole day and night showed a notable preference for the less healthy foods than the members of the well-rested group.

The other study was carried out at the University of Columbia. They also used MRI scans to monitor blood flow in the brain, to compare the brain activity in 25 volunteers, some after a normal night's sleep and others after sleeping only four hours. Both groups of volunteers were show photos of healthy foods, fruit and vegetables, interspersed with other less healthy options.

The cerebral networks linked to desire showed far greater activity amongst the volunteers who had not slept well than amongst those who had slept well, especially when they were shown photos of junk food.
 
Invesitgators say that those who had slept badly craved pizza and hamburgers because their bodies and their brains were depserately searching for the energy they lacked to face the day ahead.