People who have a poor night's sleep are more likely to crave junk food, a new study has found.

According to US scientists, previous research has suggested a link between poor sleep and obesity. They decided to investigate this further. They carried out brain scans on 23 young adults, all of whom were considered healthy.

Brain scans were carried out after a good night's sleep and then a night when sleep was disturbed.

The scans revealed that when a person had not had enough sleep, activity in the frontal lobe of their brain was impaired. This part of the brain is charged with making complex decisions.

Furthermore, activity in the deeper part of the brain was increased - this deals with how we respond to rewards.

"What we have discovered is that high-level brain regions required for complex judgments and decisions become blunted by a lack of sleep, while more primal brain structures that control motivation and desire are amplified," explained Prof Matthew Walker of the University of California, Berkeley.

In basic terms, this meant that people who had a poor night's sleep tended to crave unhealthy foods.

"High-calorie foods became significantly more desirable when participants were sleep-deprived. This combination of altered brain activity and decision-making may help explain why people who sleep less also tend to be overweight or obese," Prof Walker said.

Previous research has indicated that people who do not sleep well tend to have bigger appetites, especially for salty and sweet foods. These findings point to a particular brain mechanism controlling this, Prof Walker noted.

The results ‘shed light on how the brain becomes impaired by sleep deprivation, leading to the selection of more unhealthy foods and, ultimately, higher rates of obesity', the scientists said.

However, they added that the findings indicate that getting a good night's sleep ‘is one factor that can help promote weight control by priming the brain mechanisms governing appropriate food choices'.

Details of these findings are published in the journal, Nature Communications.