Snack and deliver: Midwives on a night shift at the Royal Sussex Hospital, Brighton / Jean-Luc Brouard

Midnight feasts lose their lustre when you're a shift worker in need of a meal

It is 7.30pm and on the 13th floor of the Thomas Kemp tower at the Royal Sussex Hospital in Brighton, eight midwives wearing royal-blue scrubs are being briefed ahead of their nightshift. Their lunchboxes put to one side, they sit where they can, sharing chairs and squeezing on to a desk. "Up here, we have the most amazing views all around Brighton," one of the midwives, 28-year-old Maggie Myatt, says. "We see the sunrise and the sunset. It might sound cheesy, but it's lovely delivering babies as the sun comes up."

It's rewarding work, but as with the rest of Britain's army of 3.6m night-shifters, the midwives at The Royal Sussex face a predicament: what do you eat to keep going through the small hours? It is an important question. A growing body of research is finding that the irregular eating patterns associated with night shifts can contribute to a number of health problems, including obesity, diabetes and heart attacks.

The midwives I speak to share in the sentiment that food serves a different purpose at night. During the day, lunch is a chance to relax and chat. But at night, eat ing is not a social occasion. You do it only for the energy it gives you. It's just fuel. But is it fuel with a downside because of the time of night it is consumed?

The midwives graze on the 'three C's': caffeine, carbs and chocolate (Jean-Luc Brouard)

You are what you eat, goes the maxim. But perhaps it is more a case of you are when you eat. Breakfast is the most important meal of the day, they say. No carbs after dark. Eat seven small meals, have a weekly "cheat day", eat only green things on a Tuesday. The restaurateur Richard Caring, constantly surrounded by delicious things, says he tries not to let solids pass his lips until the evening. There are other aspects to the question of what we eat and when. In Alan Hollinghurst's novel The Line of Beauty, the protagonist Nick Guest observes that a date's working-class sensibilities manifest themselves in eating "tea" early. Many continentals shudder at the thought of dining before nine.

I'm yet to hear of anybody who advocates eating through the small hours. From sun-down to sun-up is time for the body to indulge in R&R. Dr Simon Archer of the University of Surrey's Sleep Research Centre, says: "Your biology is set up to deal with food during the day and at night, when the restorative processes take over, to get rid of stressful toxic chemicals that have built up."

At night, eating is not a social occasion - you do it only for the energy it gives you (Jean-Luc Brouard)

Dr Archer co-authored a report last year into the health impact of working nights. It concluded that a changing shift pattern can throw a person's body "into chaos". We all have a "master clock" built into our hypothalamus that regulates the body. But each major organ and tissue also has its own local clock. They should all synchronise but by working nightshifts, we take a sledgehammer to this finely tuned set-up. While the brain senses we are awake, the liver and the digestive tract may flick over to the cleaning processes, and spurt out the chemicals, reserved for night time.

"All major organisms that have been studied have this 24-hour clock," Dr Archer says. "If you look at gene expression, you see these lovely 24-hour oscillations. Certain things get turned on during the day, and certain things go on during the night."

The office on the delivery suite is usually laden with snacks (Jean-Luc Brouard)

Of course if the world followed recommended eating patterns it would grind to a halt. Aboard a Centrica Energy natural-gas platform, 27 miles out to sea in Morecambe Bay, night shifts hinge around a meal served between midnight and 1am. In such an environment, concentration levels can't be allowed to slip. "When you're outside, especially in winter, you burn up the calories," says Stuart Jackson, the rig's Offshore Installation Manager, who works two-week stints of 6pm to 6am.

At the Royal Sussex Hospital, Maggie Myatt typically bookends a 12 and a half-hour night shift with a large dinner and breakfast back at home before bed. Through the night, she says she has no desire to eat a full meal. Instead she and the other midwives graze on the "three C's": caffeine, carbs and chocolate. Or, as Myatt puts it: "Crap, crap, and more crap". The office on the delivery suite provides a short cut from one side of the ward to the other. A desk runs down one of its walls – and it is usually laden with home-baked cakes and open packets of sweets and crisps. Midwives grab what they can. At around 4am on some nights, someone will stack a plate with white toast dripping with butter. And for when her blood sugar-level dips, Myatt will keep a packet of Haribos tucked in her pocket. Domino's pizza deliveries are not unheard of, although last orders is 11pm. After this, an auxiliary worker might be dispatched on his moped to a nearby 24-hour McDonald's.

A growing body of research is finding that the irregular eating patterns associated with night shifts can contribute to a number of health problems, including obesity, diabetes and heart attacks (Jean-Luc Brouard)

Our craving for high-calorie food when tired, says Dr Archer, compounds the issue. It is likely triggered by our body compensating for low-energy levels but we are even less adept at clearing fat at night. He believes that these factors combined explain the "strong links with obesity and subsequent diabetes and cardiovascular problems". One midwife tells me that she loses weight after a short time off work – despite eating no less.

Flight attendants could well be the people best versed in eating at strange hours. Laura Hutchinson, Flight Services Manager at Virgin Atlantic, sees time zones as the rest of us see clean clothes. The key to slipping into new ones, she says, is to drink plenty of water, get fresh air, exercise when you can – and listen to your body. "If I'm not hungry I won't eat a big meal, just because it's 'dinner time' where I am," Huthinson says. "I also have to be careful not to eat six meals in one day if I have a late start and then am trying to stay up when I get to the destination to adjust to the time zone."

The midwives at The Royal Sussex face a predicament: what do you eat to keep going through the small hours? (Jean-Luc Brouard)

When I ask Myatt if she's concerned about the long-term health effects of shift work, she pauses for a moment. Sure, yes – but there's a job to do. Her short-term complaints – bad skin, bags under her eyes and lethargy – could all be put down to lack of sleep. But one effect seems unmistakeably to do with diet. "We get nightshift belly," Myatt says. "You end up really bloated; you can leave looking more pregnant than the women."

Handover complete, the midwives head to meet their charges. In a quiet staff room just off the ward, a window looks out to the west. The sun is setting over the sea and golden light catches on the waves. Observing the scene, it strikes me that our internal rhythms seem, by definition, personal. But in a way they are also part of the same planetary routine that will see the sun come up tomorrow and the tides continue to rise and fall.