So if something is stealing your energy here are the usual suspects you should round-up:
Caffeine: the queue at the coffee bar after lunch makes it clear this is the first thing city folk reach for to get them through the slump. However after the initial buzz caffeine actually induces a blood sugar low making you feel more tired later in the day. It also depletes the body of B vitamins which are essential for energy production, so over time drinking coffee or tea will make you increasingly tired. Some people are so immune to coffee that it just makes then feel slightly less tired - this is a bad sign and means you really should give it up. Although as caffeine is a drug going cold turkey isn't advised - gradually cut the volume and switch to less draining green and white teas and every time you get a craving drink a large glass of water - it can take at least two weeks to start to feel better so perseverance is key, but in the long run you'll feel so much better for not having a daily slump.
Sugar - after coffee this is the second most common thing people reach for when tiredness hits, especially if there are biscuits or chocolate lying around the office. However just like caffeine the energy boost will be temporary and in the long run eating sugar can leave you feeling more fatigued and lethargic. It's also highly addictive so it's easy to get into a sugar cycle where you need a daily hit. You might not think you're in this cycle, but if you read the ingredients list on most foods you'll spot sugar in there, especially most snacks available in the office vending machine/coffee bar. This is why having healthy snacks in your desk drawer is essential and where Graze boxes come into their own! Having oat or rice cakes to hand is also a good idea if you need some carbs - dip in hummous or spread with non-sugar nut butter.
Wheat/gluten - if you have a gluten intolerance, like I do, then eating some regular bread or pasta can have you dozing off in minutes. But wheat can have a soporific effect even on those who aren't intolerant so if you can avoid bread at lunchtime you should - EAT sells wheat-free sandwiches and wheat-free bread to dip in your soup and Leon and pod have plenty of wheat-free options. Sushi is also readily available, but can be laced with sugar and is best balanced with some extra sashimi and edamame.
Refined carbs - even if you choose carbs that are gluten free a big plate of white rice or potatoes can easily send you to snoozy town. If you're guilty of carb overloading then limit yourself to eating the same volume of carbs as of proteins to keep you on an even keel.
Meat - meat takes a lot of energy to digest and a lot of people eat their lunch hunched over at the desks or whilst multitasking inhibitting their digestion. Stress in general suppresses the production of stomach acid and proper digestion of proteins, so hard to digest proteins like meat can divert energy from brain function to digestive function - leaving you glassy eyed. Choose easier to digest options such as eggs, pulses and fish, and keep portions of meat small at lunchtime.
Over-eating - however healthily you eat if you eat a massive portion of food your digestive tract is going to struggle to digest it diverting blood flow to your gut. It's much better to have a lighter lunch with a morning and afternoon snack if you feel peckish to keep up your energy.