It’s late, you’re exhausted, and you’re craving chips. And pizza. Or any other number of junk foods; a salad just won’t cut it. Most of us have experienced the midnight munchies, but what makes fatty, salty and sweet foods particularly appealing late at night?

Why do we crave the worst foods in the smallest hours? Image credit: KatiesCameraClicks via Flickr

Late-night cravings

If you often find yourself craving junk food at night, you’re not alone. A couple of years ago, Jawbone, the company behind a well-known fitness tracking device released summary data about the food choices their users were making. You could have easily predicted what they found. For example, people choose milk or yoghurt rather than vegetables for breakfast. Consumption of veggies peaks at dinnertime, but drops dramatically after 8 pm. At the same time, foods high in fats and sugars are strongly preferred between 8 pm and about 4 am.

Which leads to an obvious question. Does being tired (which is likely if you are up in the wee hours) cause people to crave fatty or sugary food? Many studies have shown a link between sleep deprivation and obesity, but that doesn’t tell us whether being short on sleep actually leads us to eat more junk food.

There have been many different explanations for the link between obesity and lack of sleep. For example, we know missing out on sleep disrupts the hormones that control our appetite. There’s also the simple fact that the less time you spend sleeping, the more time you can spend eating. And if you aren’t getting much sleep it’s likely you feel too tired to head to the gym. We also know drinking alcohol, which people are more likely to do at night, leads to eating more. But is there a more direct link between being sleep-deprived and craving fast food?

Off to the sleep lab

Research published this month specifically tested whether a person who is sleep-deprived eats more fatty and sugary snacks than when the same person has had enough sleep. Healthy volunteers aged between 18 and 30 each spent two four-day visits staying at a university lab. During both visits, the volunteers ate identical meals, at 9 am, 2 pm and 7 pm. During one visit, they slept an average of 7.5 hours each night. But during the second stay, they weren’t allowed to stay in bed long and slept on average only 4 hours and 11 minutes per night. On the fourth night of each stay, the volunteers were offered a range of snacks.

When the study volunteers were sleep-deprived, they binged on fatty and sugary snacks (think lollies, chips and ice cream) and ate an average of 300 extra calories. Three hundred calories is way more than needed to compensate for the extra hours of being awake: you only need about 17 extra calories for each additional hour you are up and about. That’s the equivalent of less than half an apple! But those who took part in the study said when they were sleep-deprived, they felt hungrier and found it very hard to resist the high-fat snacks on offer (there were also healthy options available).

Marijuana munchies – without the marijuana

The researchers analysed the volunteers’ blood to see if levels of particular appetite-related hormones could explain the increase in snacking. They also looked at levels of chemicals called endocannabinoids (named after cannabis, the plant that led to their discovery). The endocannabinoid system involves receptors in the body that affect the immune system and the regulation of appetite hormones. And it’s this system that is directly affected by marijuana, explaining the infamous ‘marijuana munchies’.

When sleep-deprived, the volunteers had much higher levels of a particular endocannabinoid called 2-AG that increase the pleasure we get from eating sugary and fatty foods. And the daily rhythm of 2-AG was different: when the volunteers were low on sleep, the chemical remained at high levels in the body until about 9pm, rather than until 2pm, which is normal.

What does all that mean? Simply that when you’re sleep-deprived, you crave fatty and sugary foods, and you get a lot of pleasure from eating them. It doesn’t matter if you’ve just eaten a large dinner a few hours earlier; you’re going to find it very hard to resist pizza, chocolate biscuits and ice cream.

Does it matter if you binge on junk food at night? If you’re trying to lose weight, scoffing biscuits in the wee hours is probably not ideal. But there’s another reason to shut the fridge and go to bed. Research in mice suggests eating at a time you would normally be asleep may eventually lead to difficulties in both learning and in storing long-term memories.

Hmm, where did I put that packet of chips anyway?

Links and stuff

• The Atlantic: the science of the midnight snack
• The science of how marijuana gives people the munchies
• More science on marijuana and appetite