The diver’s reflex, also known as the mammalian dive reflex and diving bradycardia, is a series of physiological responses caused by holding your breath and being submerged.

When first submerged you will feel good and relaxed, but after about 30 seconds you will start to feel the buildup of CO2 in your lungs. You’ll feel the need to take a breath, but you’ll still have air. The uncomfortable feeling is just CO2.