How much water should you drink after a sauna?

Water is not enough – what you lose in the sauna and how to replace it properly

You drink water. You sweat. You drink more water. And yet sometimes you feel completely wrung out after the sauna. The problem is not your water intake – it is what you forget to replace.

There is a very common misunderstanding about sauna bathing and hydration: that thirst is your guide and water is the answer. That is partly true. But anyone who uses a sauna regularly – and wants to get the most from it – needs to understand that sweat is not the same thing as water.

Sweat is water plus electrolytes. And when you replace sweat losses with water alone you can actually make the situation worse, not better. This is called dilutional hyponatraemia – too little sodium in the blood – and it can produce exactly the symptoms people wrongly attribute to dehydration: headache, dizziness, fatigue and muscle cramps.

The good news: getting it right is not complicated. You just need to understand what the sauna actually does to your body – and adjust your fluid intake accordingly.

Sweat is not just water – what actually leaves the body in the sauna

When you sit in a sauna your body temperature rises and the sweat glands activate to cool you down. An average person loses 0.5 to 1 litre of sweat per 30-minute session in a traditional sauna. With löyly – when you pour water on the hot stones and the steam intensifies – sweating increases markedly in a short space of time.

But it is not only fluid that leaves the body. Sweat contains:

• Sodium and chloride – around 1 gram of sodium per litre of sweat. It is the electrolyte you lose most of, and the one that matters most for how you feel afterwards.

• Potassium – important for muscle contraction and heart rhythm. Lower losses than sodium, but relevant in longer sessions.

• Magnesium – muscle relaxation and recovery. Losses are lower per session but accumulate with frequent sauna use.

• Trace amounts of heavy metals – lead, mercury and BPA leave the body via sweat. One of the genuine detox effects of regular sauna bathing that has scientific support.

How much do you actually sweat in the sauna – and how does a traditional sauna differ from an infrared one?

It is a question most people never think about – but the answer decides how you should adjust your fluid intake.

A traditional sauna at 80–100 °C creates fast, heavy sweating. That is the acute, intense response – the body mobilises every cooling system at once. Löyly amplifies the effect further: when you pour water on the stones the perceived temperature rises within seconds, pores open faster and sweating intensifies. A session with löyly can produce noticeably greater sweat loss than a dry session at the same temperature.

An infrared sauna at 40–60 °C produces a different kind of sweating. It is deeper, more gradual and more drawn out. The lower air temperature means total sweat volume per session is somewhat lower – but the session is longer, 20–30 minutes versus 15–20 minutes, which makes total loss more comparable than you might think.

The practical advice: adjust your fluid intake to session length and intensity, not just to the type of sauna. A 30-minute session with heavy löyly needs more replacement than a gentle 20-minute infrared session.

A hydration protocol for sauna users – what to drink before, during and after

A study published in Annals of Clinical Research established the most important thing that can be said about sauna bathing and fluid balance: replace losses early, in small doses and often. Never go into the sauna already thirsty. And if you have used the sauna intensively after training – start replacing fluid and electrolytes before the sauna session.

The simple weighing trick: weigh yourself before and after a sauna session. 1 kg lost = roughly 1 litre of fluid. Minimising the weight loss is not a sport – it is a way of genuinely understanding how much you lose and adjusting your recovery accordingly. Most people are surprised the first time they try it.

When is water enough – and when do you need electrolytes?

The most common mistake is buying electrolyte products for the wrong reason – or skipping them entirely when they would genuinely make a difference. Here is the simple version:

Water is enough when:

• Your session is short, 15–20 minutes, at a normal temperature without intense löyly

• You eat normally, with salty food during the day

• You did not train shortly before the sauna

• You are not in a hot climate that increases sweat loss further

Electrolytes make a difference when:

• Your session is long – 30 minutes or more, or several rounds

• You use the sauna straight after training

• You do contrast therapy with several rounds of sauna and cold plunge

• You eat a low-carbohydrate diet – the body retains less sodium without carbohydrates

• You notice symptoms: headache, cramps, dizziness or extreme fatigue after the sauna

• You use the sauna 4–7 times a week – high frequency causes cumulative electrolyte loss over time

Contrast therapy and hydration – the part nobody talks about

This is where most people doing contrast therapy make their biggest mistake.

When you step out of the sauna and into a cold plunge, something happens that fools the brain: you feel immediately alert, sharp and recovered. Noradrenaline shoots up, the body's cooling systems kick in and the warm, tired feeling disappears within seconds. It feels as though the body has been reset.

But the sweat loss from the sauna is still unreplaced. The cold plunge is not a magical restoration of electrolytes – it is a physiological stimulus that masks the signals telling you that you still need to replace what you lost.

A practical rule for contrast therapy: replace fluid and electrolytes based on the sauna session, not on how you feel after the cold plunge. If you do three rounds of sauna and ice bath over 90 minutes you have sweated heavily – however fresh you feel when you step out of the cold water at the end.

Drink water and eat something salty – or have an electrolyte drink – within 30 minutes of a complete contrast therapy session. Do not wait until thirst kicks in, because it can be delayed.

Read the signals from your body – six signs to recognise

You do not need a laboratory to work out whether you are handling hydration properly. The body gives clear signals – if you know what to look for:

Summary – the simple things to remember

Hydration around sauna bathing does not have to be complicated. But it does take a little more thought than simply drinking water.

• Drink water 30–60 minutes BEFORE. Never go in thirsty.

• Sweat contains electrolytes, not just water. Sodium is the most important – around 1 gram per litre of sweat.

• A traditional sauna makes you sweat more per minute than an infrared one. Löyly intensifies the loss further.

• Replace gradually after the session. 1 kg of weight loss = roughly 1 litre of fluid to replace.

• Contrast therapy tricks you into feeling recovered. Replace fluid based on the sauna session, not on how alert the cold plunge makes you feel.

• Electrolytes for a long or intense session. Water plus normal salty food is enough for shorter sessions.

• Frequent sauna use needs more attention. 4–7 sessions a week cause cumulative electrolyte losses that need active replacement.

If you use a Swedish Cold sauna – whether that is a traditional sauna, an infrared sauna or a hybrid sauna – and combine it with an ice bath, remember that your body is working hard during the session. Give it what it needs to recover properly.

Sources:

1. Annals of Clinical Research / PubMed (1988) – Sauna and body fluid balance: fluid, electrolyte and acid-base balance during and after sauna bathing

2. Harvard Health – How much water should you drink?

3. Healthline – Electrolyte water: benefits and myths

4. Cleveland Clinic – Hyponatraemia: too little sodium in the blood

5. Huberman Lab – Using salt to optimize mental and physical performance

6. Laukkanen et al., JAMA Internal Medicine (2015) – Sauna bathing and cardiovascular mortality, 2,315 participants over 20 years