People who consume a lot of fat in their diet may be more sleepy during the daytime as a result, a new study suggests.

US scientists followed the progress of 31 healthy people aged between 18 and 65. None of the participants were obese or had any kind of sleeping problems.

All slept for four nights in a special sleep laboratory. On the final day, daytime sleepiness and diet were assessed.

The scientists found that those who ate higher amounts of fat tended to be more sleepy during the day, while those who ate more carbohydrates were more alert. Protein did not appear to have any effect.

The results were irrespective of the participants' age, gender, body mass index, calorie intake and hours spent sleeping per night.

The scientists noted that excessive daytime sleepiness and fatigue ‘are very prevalent in the modern world' and eating more fat ‘has an acute adverse effect on alertness of otherwise healthy, non-obese adults'.

"It appears that a diet high in fat decreases alertness acutely and this may have an impact on an individual's ability to function and also public safety," they said.

Details of these findings are published in the journal, Sleep.