Choose egg whites on toast, rather than jam on toast, say scientists who have identified that protein not sugar activates the cells that keep us awake and burn calories.
The finding has implications for understanding obesity and sleep disorders.
Wakefulness and energy expenditure rely on "orexin cells", which secrete a stimulant called orexin/hypocretin in the brain.
Reduced activity in these unique cells results in narcolepsy and has been linked to weight gain.
Scientists at the University of Cambridge compared actions of different nutrients on orexin cells. They found that amino acids - nutrients found in proteins such as egg whites - stimulate orexin neurons much more than other nutrients.
Previous work by the group found that glucose blocks orexin cells (which was cited as a reason for after-meal sleepiness), and so the researchers also looked at interactions between sugar and protein.
They found that amino acids stop glucose from blocking orexin cells (in other words, protein negated the effects of sugar on the cells).
These findings may shed light on previously unexplained observations showing that protein meals can make people feel less calm and more alert than carbohydrate meals.
"To combat obesity and insomnia in today's society, we need more information on how diet affects sleep and appetite cells," said lead researcher Dr Denis Burdakov of the Department of Pharmacology and Institute of Metabolic Science.
"For now, research suggests that if you have a choice between jam on toast, or egg whites on toast, go for the latter! Even though the two may contain the same number of calories, having a bit of protein will tell the body to burn more calories out of those consumed," he added.
The study has been published in the 17 November issue of the scientific journal Neuron.