Craving a snack before bed? Biology explains the urge, and habits can change it

You've finished dinner and you're comfortably full. Half an hour later you're standing in front of the kitchen cupboards anyway. Research suggests this is not unusual: about a third of people snack every night after their evening meal, and among 18-to-24-year-olds the figure is four in five.

Dietitian Dr Kristine Beaulieu, who splits her time between the University of Leeds and the University of Moncton in Canada, says this is cause for concern. She points to evidence that the more people eat within two hours of going to bed, the greater the odds of weight gain and obesity.

Part of the explanation is that appetite appears to peak in the evening, almost regardless of how much a person ate earlier in the day, even though the hours spent asleep are a long stretch without food. Prof James Betts, who studies how the body handles food and energy at the University of Bath, says the hunger hormone ghrelin is generally higher later in the day. Meals tend to suppress it, he says, but that really only happens after the first meal of the day, not after lunch or dinner.

What's on the plate matters too. Meals rich in protein, fibre and volume tend to keep people satisfied for longer. Betts says an evening meal that isn't nutritious or filling can lead to a later crash, and that's when people snack. For some, a fear of waking hungry in the night adds to the pull, and habit plays its part. Prof Suzanne Higgs, an appetite expert at the University of Birmingham, says that if snacking usually happens in one setting, the surrounding cues can start to trigger cravings. Someone who always snacks in front of the television, she says, may feel like eating simply because the set has been switched on.

The pull toward sweet and fatty food is its own thing. Research suggests the appetite for calorie-dense foods grows stronger later in the day, driven by the brain's reward system rather than a real need for energy. Beaulieu says liking and wanting for high-fat foods are greater in the evening than for low-fat ones, and that this hedonic drive can override signals of fullness, so a person wants dessert even when full. Foods rich in sugar, fat or salt can keep activating reward pathways even after a substantial meal. Standing at the biscuit tin at 9pm, in other words, may not reflect a lack of willpower; the biology is stacked against you.

Whether snacking actually harms sleep is less settled. Eating a large meal late affects digestion and the body's normal preparation for sleep. For snacks, the research is murkier. Some evidence links refined carbohydrates and added sugars, from white bread and white rice to biscuits and cakes, with poorer sleep. Researchers think this could be because such foods cause bigger swings in blood sugar and may alter hormonal signals that regulate sleep and appetite. But cause and effect is hard to separate: Prof Jonathan Johnston, a sleep and body clock expert at the University of Surrey, notes that sleep deprivation increases appetite and the desire for energy-dense foods. Betts says it is very difficult to draw firm conclusions, and that people may simply respond differently to different foods before bed.

The advice starts with working out what is driving the snacking. If it is real hunger, making earlier meals more filling may help. If it is mostly habit, changing routines and planning snacks in advance can help. Higgs adds that eating while distracted, such as in front of the TV, can lead people to eat more because they pay less attention to how much they've had.

Nobody quoted calls for a ban. Beaulieu says a reasonable portion of something enjoyable is often more sustainable, that fruit, yoghurt and nuts should be kept on hand, and that even a little chocolate now and then is not the end of the world. Her trick is to make the healthy choice the default: if craved foods aren't regularly in the cupboard and a healthy option is, you are more likely to pick the healthier one.