A lack of sleep not only increases tiredness and irritability - it can also expand your waistline.After being deprived of one night's rest, participants in a study went on to purchase food that was higher in calories and weighed almost a fifth more than their normal shop.Scientists from Uppsala University in Sweden found that poor or no sleep resulted in raised levels of a hormone that is linked to boosting hunger.Instead, the findings suggest another mechanism, such as reduced willpower or impulsive decision making, may be to blame.In the tests, adults who spent an entire night with no sleep were given a selection of 40 food items the following morning.They bought food with nine per cent more calories compared to mornings when they had a night's sleep.Shopping while tired could have a knock-on effect for our overall wellbeing, the researchers warned, as individuals will often purchase several meals during one trip.'We hypothesised that sleep deprivation's impact on hunger and decision making would make for the 'perfect storm' with regard to shopping and food purchasing-leaving individuals hungrier and less capable of employing self-control and higher-level decision-making processes to avoid making impulsive, calorie-driven purchases,' said lead author Colin Chapman from Uppsala University in Sweden.