Why does my CGM show a blood sugar spike after hot shower?

Hi Mark, this is actually a very common phenomenon, especially for people who wear continuous glucose monitors (CGMs). When you take a steamy shower or soak in a hot bath, your body reacts to the heat in a few different ways. First, the heat can act as a physical stressor, which sometimes triggers a mild release of stress hormones like adrenaline and cortisol. These hormones signal your liver to release stored glucose into your bloodstream, causing a genuine, temporary rise in your numbers.

However, there is also the technology side to consider. CGMs read the interstitial fluid under your skin, not your actual capillary blood. Some research and user experiences suggest that extreme changes in temperature can cause this fluid to flow more loosely, which can trick the sensor into reporting a "false high".

For most people, this post-shower spike is temporary and settles down within 30 to 60 minutes once the body cools off. A practical way to figure out what is happening is to do a quick fingerstick with a standard blood glucose meter the next time your CGM alerts you in the bathroom. If the fingerstick shows a normal number, it's just the sensor reacting to the heat. To manage the annoyance of the alarms, some people prefer taking lukewarm showers or simply making a mental note in their logbook to ignore that specific 20-minute spike window.