There are many biological reasons as to why exercise might make us feel a bit worse for wear, according to Dr Dehghan.

Lost energy and dehydration

“Firstly, during exercise, the glycogen stores in the muscles get depleted. When that happens, the body struggles to maintain blood sugar levels which can result in many feeling light-headed and tired. At the same time, we lose fluid through sweating and, with that, salts. This can cause a drop in blood pressure which can leave people feeling dizzy,” she says.

Build up of lactic acid

“The feelings of nausea and lethargy can also be caused by a build-up of lactic acid in muscles. During training, we need more oxygen to break down glucose and produce energy. However, during a strenuous training session, our body cannot meet the increased demand for oxygen and that leads to the production of lactic acid – which can make people feel nauseous, weak, have stomach pain or cramping, burning sensations in the muscles.”

Lack of oxygen to the stomach muscles

Other research suggests it’s down to the conflict in the body from different tissues all demanding oxygen when you work out. For instance, the blood vessels to less active tissues, such as the stomach, are narrowed, while blood flow to the working muscles and the heart is increased. The change in demand as well as the ‘switching off’ of the gastrointestinal tract can cause nausea.

Why do I feel sick after workout out? Post-exercise nausea explained