Should You Drink Water in a Sauna?

Drinking water before, during, and after a sauna session is one of the clearest pieces of health advice around this practice. A single session can strip anywhere from about half a pound to over two pounds of body weight, almost entirely as sweat, and that fluid needs replacing. The question isn’t really whether to drink water in a sauna but how much and when, and what happens if you don’t.

How Much Fluid You Actually Lose

The amount of sweat you produce in a sauna varies more than most people expect. One study measuring body mass loss in a dry sauna found individual losses ranging from 0.2 kg to 1.1 kg in a single session, with an enormous spread between people even when session length and temperature were the same.¹ That’s roughly half a pound at the low end and nearly two and a half pounds at the high end, all water. People with higher body mass tend to sweat more and face a greater dehydration risk, so body size matters.²

This variation helps explain why a blanket recommendation like “drink one glass of water” is too simple. Two people sitting side by side in the same sauna at the same temperature for the same 20 minutes might lose dramatically different amounts of fluid. The coefficient of variation in body mass loss across sauna users in that study was over 45%, far higher than the variation in any other measure recorded.¹ Your personal sweat rate depends on your fitness level, body composition, how heat-adapted you are, and even genetic factors.

What the Heat Does to Your Cardiovascular System

Understanding why hydration matters in a sauna goes beyond “you sweat a lot.” When your body heats up in a sauna, your blood vessels near the skin dilate to push warm blood toward the surface for cooling. Heart rate climbs, cardiac output increases, and blood pressure tends to drop modestly. The net effect is a state researchers describe as transient hypovolaemia, meaning you have slightly less circulating blood volume than normal because fluid has shifted outward through sweating and vasodilation.³

Heart rate during a sauna session can rise substantially. The same body-mass-loss study found heart rate increasing by an average of about 57 beats per minute over the course of the session, which is roughly a 72% jump from resting.¹ Your heart is working harder to circulate a shrinking volume of blood. Every glass of water you drink helps maintain that blood volume and takes some of the strain off your cardiovascular system.

Hormonal Shifts and Electrolyte Changes

Your body doesn’t just lose water in the sauna; it activates a cascade of hormonal responses to manage the stress. Research on short, intense sauna exposure found that serum aldosterone, the hormone that tells your kidneys to retain sodium and water, rises during a session. Growth hormone and prolactin also spike. Interestingly, antidiuretic hormone, which you might expect to climb when the body is losing fluid, stayed unchanged in this particular study, as did plasma sodium levels and catecholamines.⁴

The aldosterone increase is your body’s attempt to hold on to what fluid it has left. But this mechanism can only do so much if you keep sweating without replacing what’s lost. This also raises the question of whether plain water is always enough. For a typical 15-to-20-minute session, water alone is fine for most people. If you’re doing repeated rounds or spending longer than 30 minutes total in the heat, the sodium you lose in sweat starts to matter, and something with electrolytes can help your body absorb and retain the fluid more effectively.

When to Drink and How Much

The practical advice boils down to hydrating in three phases: before, during, and after. Drinking a couple of glasses of water in the 30 to 60 minutes before your session means you start with a reasonable fluid reserve. Taking small sips during the session, rather than chugging a large volume at once, keeps your stomach comfortable in the heat. And replacing whatever you lost afterward is the final piece.

A useful rough guide is to weigh yourself before and after, if you have the means. Every kilogram lost is roughly a liter of fluid to replace. Since individual losses vary so widely, this is far more accurate than any fixed recommendation. If weighing yourself isn’t practical, aim to drink at least two to three glasses of water for a standard 15-to-20-minute session, and more if you’re doing multiple rounds.

One thing worth knowing is that dehydration itself slows down how quickly your stomach empties. A study measuring gastric emptying found that when people were already dehydrated, the time it took for fluid to move from the stomach into the intestines was significantly longer compared with when they were well-hydrated.⁵ In practical terms, this means waiting until you’re already thirsty and dried out before you start drinking is a losing strategy. The fluid you drink in a dehydrated state takes longer to get where it needs to go, which is exactly when you need it fastest.

Why Thirst Is an Unreliable Signal

Most people assume they’ll simply feel thirsty when they need water. In a sauna, this is a particularly bad assumption. Thirst is a lagging indicator, not a leading one; by the time your brain registers a strong thirst signal, you’re already meaningfully dehydrated. And for some groups, the signal is even weaker than usual.

Older adults face a compounded problem. Research on heat acclimation in aging populations has shown that older men have both a slower sweat response and a blunted thirst response compared with younger men. On top of that, older adults have a decreased ability to recover from dehydration once it sets in.⁶ The combination of sweating less efficiently, noticing thirst less readily, and recovering more slowly makes proactive hydration especially important for anyone over about 60. Researchers studying heat protocols for older adults have specifically recommended ad libitum hydration with a minimum consumption threshold and active encouragement to drink, rather than relying on thirst alone.⁶

This doesn’t only apply to the elderly. Anyone taking medications that affect fluid balance, like diuretics or certain blood pressure drugs, should approach sauna hydration more deliberately. If you’re on medication that makes you urinate more or that lowers blood pressure, the combined effect with sauna-induced fluid loss and vasodilation can drop your blood pressure to uncomfortable or unsafe levels.

The Alcohol and Sauna Problem

In many sauna cultures, beer and sauna go hand in hand. The research suggests this is genuinely dangerous, and the mechanism involves the same fluid dynamics that make hydration so important in the first place. Alcohol is a vasodilator, and so is sauna heat. Combine the two, and blood pressure can plunge. One study found that sauna bathing alone kept systolic blood pressure roughly at baseline, but sauna combined with alcohol dropped it from about 136 mmHg to 113 mmHg, a steep and clinically meaningful fall.⁷

The drop in blood pressure creates a risk of orthostatic hypotension, where standing up suddenly causes dizziness or fainting. In a hot, slippery sauna environment, fainting is a serious injury risk on its own. But the problems go deeper. Alcohol intoxication and particularly the hangover phase expose a person to cardiac arrhythmias, and the increased adrenergic activity during sauna bathing can amplify that risk further.⁸ The rare cases of sudden cardiac death linked to sauna use have been attributed not to the sauna itself but to the combination of dehydration, low blood pressure, and arrhythmia triggered by alcohol consumption alongside the heat.⁹

Alcohol also acts as a diuretic, accelerating fluid loss through urination on top of what you’re already losing through sweat. Replacing sweat losses with beer is essentially replacing water you’ve lost with a liquid that makes you lose even more. If you enjoy a drink after the sauna, do it after you’ve rehydrated with water first, and ideally after you’ve cooled down.

Traditional Saunas Versus Infrared Saunas

Hydration advice applies to all types of saunas, but the intensity of the cardiovascular demand differs between them. Traditional Finnish-style saunas operate at higher temperatures, typically well above 80°C, with varying levels of humidity. Far-infrared saunas work at considerably lower air temperatures, often in the range of 35 to 50°C. A study comparing the two found that heart rate after a traditional sauna session averaged about 92 beats per minute, while after a far-infrared session it was around 71, a meaningful difference.¹⁰

The lower cardiovascular stress of infrared saunas doesn’t mean you can skip hydration. You still sweat, and the lower perceived intensity may actually make you stay in longer, which can increase total fluid loss even though the minute-by-minute rate might be lower. But if you’re new to sauna use, have cardiovascular concerns, or simply dislike the intense blast of traditional saunas, infrared versions offer a gentler entry point where the hydration demands are somewhat reduced for a given session length.

Sauna Dehydration and Physical Performance

Athletes and combat sport competitors sometimes use saunas to cut weight quickly before a weigh-in. The physiological cost of this practice is worth understanding even if you never compete, because it illustrates just how much sauna-induced dehydration can impair your body. In a study comparing different dehydration methods, participants who lost about 4% of their body weight through sauna bathing saw meaningful drops in maximal oxygen consumption, cardiac efficiency, and the amount of work they could produce at peak effort.¹¹

The same study found something striking: sauna-induced dehydration was more harmful to performance than the same amount of weight loss achieved through exercise. The researchers concluded that both the amount of weight lost and the method of losing it matter. Rapid fluid stripping through heat exposure, without any of the physiological adaptations your body makes during exercise, appears to be especially punishing. A more gradual weight loss over 48 hours was less damaging than the same loss compressed into 24 hours via sauna.¹¹

For recreational sauna users, the takeaway is simpler: if you’re planning to exercise after a sauna, or if you use the sauna as part of a workout routine, you need to be fully rehydrated before you train. Even modest dehydration of a percent or two of body weight can noticeably affect endurance and coordination. Sitting in a sauna for 20 minutes without drinking and then jumping on a bike or heading to the gym is asking your heart and muscles to perform under a real handicap.

Higher-Risk Groups and Practical Adjustments

Beyond older adults, several groups should approach sauna hydration with extra care. People with higher body mass lose more fluid per session and face a proportionally greater dehydration risk.² Anyone with kidney disease may have impaired ability to concentrate urine and retain fluid. People with diabetes, particularly those on medications that affect fluid balance, should consult their physician about sauna use. And anyone recovering from illness, diarrhea, or vigorous exercise is already starting from a fluid deficit that the sauna will worsen.

Pregnant individuals are generally advised to avoid saunas or limit sessions to short durations at lower temperatures, primarily because of concerns about core temperature elevation rather than hydration per se. But fluid loss is an added consideration, since blood volume demands are already elevated during pregnancy.

A few practical habits make a real difference regardless of your risk profile:

Pre-hydrate: Drink at least one to two glasses of water in the hour before you enter.
Bring water in: Keep a bottle within reach and take small sips throughout, rather than waiting until you feel thirsty.
Monitor your urine: If your urine is dark yellow or you haven’t urinated for several hours after a session, you haven’t replaced enough fluid.
Limit session length: First-timers and people in higher-risk groups should start with 10 to 15 minutes and build up gradually.
Cool down before rehydrating heavily: Give your body a few minutes to begin cooling before drinking large volumes, which helps avoid stomach discomfort.

What About Sports Drinks and Coconut Water

For most casual sauna sessions, plain water does the job. Sweat contains sodium, potassium, and small amounts of other minerals, but a typical 15-to-20-minute session doesn’t create losses large enough to worry about for a person eating a normal diet. Your next meal will replenish what was lost.

The calculus changes with longer or repeated sessions. If you’re doing the classic Finnish pattern of multiple rounds with cold plunges in between, or if your total heat exposure exceeds 30 to 40 minutes, an electrolyte drink can help. The aldosterone spike your body produces during sauna exposure tries to hold onto sodium, but it can’t fully compensate for heavy or prolonged sweating. A drink with some sodium also promotes better fluid retention than plain water, because sodium helps your intestines absorb water and signals your kidneys to hold on to what you’ve consumed rather than letting it pass straight through.

Coconut water, diluted fruit juice, or commercial electrolyte tablets dissolved in water all work. Sugary sports drinks aren’t ideal since the high sugar content can slow absorption and cause stomach discomfort in the heat. The goal is modest sodium and potassium without a lot of extra calories or sugar. If you prefer to keep it simple, a pinch of salt in your water bottle after a long session accomplishes much of the same thing.

1
Revista Brasileira de Medicina do Esporte. Body Mass Loss in Dry Sauna and Heart Rate Response to Heat Stress
2
PubMed Central. Sauna-induced body mass loss in young sedentary women and men
3
Quality in Sport. Sauna, the Cardiovascular System and Autonomic Regulation: Implications for Post-exercise Recovery
4
PubMed. Children in sauna: hormonal adjustments to intensive short thermal stress
5
PubMed. Effect of dehydration on gastrointestinal function at rest and during exercise in humans
6
PLOS ONE. Short-term heat acclimation protocols for an aging population: Systematic review
7
PubMed. Alcohol and sauna bathing: effects on cardiac rhythm, blood pressure, and serum electrolyte and cortisol concentrations
8
PubMed. The sauna and alcohol
9
PubMed. Is sauna bathing protective of sudden cardiac death? A review of the evidence
10
SpringerPlus / BioMed Central. Effects of far-infrared sauna bathing on recovery from strength and endurance training sessions in men
11
PubMed. Differential effects of sauna-, diuretic-, and exercise-induced hypohydration