We might start with good intentions but our willpower to resist eating cake weakens as the day goes on, scientists have found.
In the morning, we are more likely to associate cake and other less healthy food such as crisps, burgers and pizza with negative experiences such as sickness, pain, abuse fear and even death.
But by the afternoon, as our resistance slackens and our minds view 'junk food' in the same way as we see positive things such as rainbows and holidays.
Researchers from Flinders University in Adelaide, Australia, and Liverpool University carried out a psychological experiment called an 'implicit association test' on 304 women aged 17-25.
Every hour from midday, groups were shown images and words which they had to react to with either positive or negative scores. The words included death, pain and abuse as well as holidays, rainbows, peace and love.
Mingled among them were crisps, chocolate, burgers, pizzas and cake. The non-food words were generally easy to sort as either positive or negative but as the day went on the foods were gradually more likely to be classed positively.
The researchers used questionnaires throughout to record how hungry the women were and analysed which ones had previously said they were watching their weight.
The researchers believe a combination of the human body and mind as well as tradition and culture play a part.
Just as people rarely have alcohol in the morning and may smoke more at night, so they also associate unhealthy foods with later times of the day.
But in addition the body and mind may both start the day determined to eat healthily but that resistance gradually wears down as the day goes on.
The report, for the journal Food Quality and Preference, said: 'Time of day correlated with "implicit evaluations" of unhealthy snack food.
'Individuals who participated later in the day tended to implicitly evaluate unhealthy snack food as more positive than those who had participated earlier in the day.'