Sauna while fasting is a popular pairing and the honest answer is that the two stack in a way worth respecting. Both lower blood pressure, both deplete fluid and electrolytes, and the light-headedness people report is the predictable result rather than a sign that something unusual is happening.
Why the two stack
Heat dilates blood vessels and sweating reduces blood volume, both of which lower blood pressure. Fasting, particularly beyond a day or with low carbohydrate intake, also lowers blood pressure and depletes sodium as the body sheds water. Doing them together produces the classic standing-up-too-fast sensation, and occasionally a faint.
What to watch for
Light-headedness, a racing pulse, feeling cold at the edges, weakness or difficulty concentrating. Get out at the first of them rather than the third. Standing up quickly in the cabin or immediately on leaving is when people actually fall, so move deliberately and sit down outside before you stand.
If you do both
Keep it shorter and cooler than your usual session, do it earlier in the fast rather than at the end, drink water with electrolytes rather than water alone, and never do it alone. Skip the cold plunge afterwards while fasting: the contrast is a large cardiovascular demand and you are already running low on the reserves that handle it.
Who should not
Anyone with low or poorly controlled blood pressure, a cardiovascular condition, diabetes or any condition affecting fluid balance, and anyone on medication that lowers blood pressure or is a diuretic. This is a clinician's question. No claim on this page and nothing a retailer tells you replaces asking somebody who knows your history.
Questions people ask about sauna while fasting
Is it safe to use a sauna while fasting?
For a healthy person on a short fast, with a shorter cooler session and electrolytes, usually. Both lower blood pressure, so they stack.
What should I drink?
Water with electrolytes rather than water alone, because fasting sheds sodium and sweating sheds more.
Can I cold plunge too?
Better not while fasting. Contrast is a large cardiovascular demand at a time when your reserves for it are lower.