“Afterdrop” is common after swimming in cold water; you get out and feel fine, and then you start to get colder, sometimes growing faint, shivering violently and feeling unwell.
Afterdrop happens because when you swim, your body shuts down circulation to your skin, pooling warm blood in your core. This process helps you stay in the water longer: with reduced circulation to your peripheries skin and sub-cutaneous fat is turned into a thermal layer, similar to a natural wetsuit – hence the wild swimmers’ term bioprene for fat.
But as you start to warm up, this process reverses: blood starts to recirculate in your extremities and peripheral blood vessels, cooling as it travels. You can lose up to 4.5°C from your core temperature (according to Golden and Tipton, Essentials of Sea Survival), bringing on shivering, hypothermia, or feeling faint and unwell.
The key to warming up and staying well is to warm up slowly and gradually. If you attempt to rush it by, for example, having a warm shower or bath, you will draw the warm blood that has pooled in your core to the skin at speed, leading to rapid cooling. You will quite likely faint as your temperature plummets along with your blood pressure.
With help from Dr Mark Harper and Heather Massey from the Extreme Environments Laboratory at the University of Portsmouth