If you can't figure out why you're so desperately craving junk food late at night, this new research says that it might be because you're not getting enough sleep.

Based on studies from Cologne University in Germany, tiredness boosts both production of brain cells and hunger hormones that are linked to comfort eating and hunger respectively.

Additionally, researchers say that participants who went an entire night with no sleep were willing to fork out more cash for snacks like chocolates and biscuits compared to non-food items. On the other hand, they would not spend the same excessive amounts of money after getting a proper night's rest.

It explains why we are more likely to reach for the biscuit tin than a piece of fruit late at night. Britain is among the most overweight and sleep-deprived nations in the world – and evidence shows that the two are linked.

Psychologist Julia Rihm said: "We found a full night of sleep deprivation compared with a night of habitual sleep increased the subjective values of snack food rewards compared with non-food rewards."

Using MRI (magnetic resonance imaging) brain scans, her team showed losing sleep fired up neurons in the amygdala and hypothalamus. The former is an area of grey matter that has been linked to reward seeking behavior – such as eating – under stress. The latter controls appetite.

"The data suggests one way a lack of sleep can promote overeating and obesity risk," said Rihm. "This behavioral result was paralleled by increased amygdala and hypothalamus activity selectively after sleep deprivation.

Blood tests also showed increased levels of ghrelin, the hormone that tells us to eat.

The study published in JNeurosci was based on 32 healthy 19 to 33-year-old men of normal weight who were kept awake and allowed to sleep normally.

Each experiment followed an evening meal of yogurt and pasta and meat in a creamy mushroom sauce at Rihm's laboratory on two separate nights.

At each visit, they were instructed to either return home to get a normal night's sleep while wearing a watch that monitors activity or to spend the night at the lab where they were kept awake. This allowed the team to compare their desire for snack foods – along with their brain activity and hormone levels – during each morning after the experiments.

The next day, they were given €3 to spend on snacks, such as popular brands of German chocolate bars or chips, or everyday household items or university merchandise.

In an online auction, images of the goods flashed up on screen with prices going up in increments of 25 pence. Only after sleep deprivation were the participants willing to pay extra money for the junk food items, which they were then allowed to eat afterwards.

"Sleep loss is associated with increased obesity risk, as demonstrated by correlations between sleep duration and change in body mass index or body fat percentage," said Rihm. "We found ghrelin concentrations were increased after sleep deprivation compared with habitual sleep.

"Despite similar hunger ratings due to fasting in both conditions, participants were willing to spend more money on food items only after sleep deprivation.

"Furthermore, MRI data paralleled this behavioral finding – revealing how a food reward-specific up-regulation of hypothalamic valuation signals and amygdala-hypothalamic coupling after a single night of sleep deprivation."

She said the findings shed fresh light on the link between sleep deprivation and obesity.

"One problem of modern society is sleep restriction building up constantly over a week of work rather than total sleep deprivation," said Rihm. "However, in our approach, we used a total night of using sleep deprivation as a first step in unravelling mechanisms underlying sleep loss-associated increased food valuation.

"Taken together, our findings reveal a mechanism through which sleep deprivation might promote food intake by enabling food cues to gain access to processing in hypothalamic circuits via the amygdala.

A similar study conducted by Swedish researchers last year found that their participants went on to purchase food that was higher in calories and weighed almost a fifth more than their normal intake after losing one night's sleep, which implies that grocery shopping while tired could have a knock-on effect for our overall wellbeing.

So if you're anxious about your junk food intake, try hitting the hay a little earlier and see if it affects your dietary choices.

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