You're not sleeping well

When you sleep your body naturally slows down and uses less energy. You might sleep for ten hours and feel fine in the morning. But try not eating or drinking for the same amount of time. You'd be thirsty and hungry, right?

If you're having trouble sleeping, then your body tends to go into stress mode. Stress hormones such as cortisol are naturally low at night, but will go up again if we're awake. Cortisol makes our bodies lay down fat reserves 'just in case'.

Plus if you're awake, you're up and possibly in the kitchen - so your tendency to snack is higher.

The underlying question is - why aren't you sleeping?

Sarah Walford, a nutritionist based in London, explains that stress is a two-fold process.

"Some people feel so anxious they can't eat, while others tend to binge eat," she says. "When you feel stressed, your body releases cortisol in an attempt to free up sugar into your bloodstream. If you then can't burn off that sugar it gets converted to fatty acids, which makes you put on weight. That kind of weight gain is often around the tummy rather than the arms or legs."

Walford explains that, nowadays, more people work through the night.

"Think nurses, factory workers, delivery drivers, office cleaners," she says. "Your body wants to be asleep, but if you're awake and working then your cortisol production kicks in. This is the same bad cycle we see in people who are stressed or anxious. Cortisol frees up sugar which - if it isn't burnt off - gets converted to fatty acids and laid down as fat."

With more people working nights, this increases the chance of negative consequences for their weight and health.

When you work nights, you also tend to eat unhealthily and mainly during the later hours. This is bad for your health, but also tends to make you put on weight because your body can't digest food as efficiently at night as during the day.

You're going out for dinner with friends later, so what do you do for lunch? Skip it? This technique is unlikely to lead to weight loss.

Walford warns: "Skipping meals puts your body into self-defence mode. Low food intake slows your metabolism down. So then when you go back to eating normally you're putting food into your body but not burning it off as quickly."

You end up putting the weight back on and sometimes even more than before."

In most places there's plenty of coffee shops. These coffees or hot chocolates may taste nice and make you feel good - for a bit - but they aren't a great choice if you're trying to lose weight.

Studies that go back to the 1990s have consistently shown that we underestimate how much we eat.

If you're having trouble keeping track of what you eat, try snapping a photo of literally everything you eat or drink over a 24-hour period. The results might surprise you.