Dehydration
Dehydration in the sauna is the condition that results when the water lost through sweating exceeds the fluid taken in. Because a sauna session can remove a substantial fraction of a litre of water from an active bather, and because the sensation of thirst is a late signal of a developing deficit, dehydration is the commonest preventable disturbance of the body during bathing. It is closely tied to Heat exhaustion, to Heat stress in general, and, when repeated or extreme, to the function of the kidneys.
Fluid losses during sauna bathing
In the hot room the body must evaporate sweat to lose heat, and the fluid for that sweat comes from the body's water stores. Studies of Finnish sauna bathing record average sweat-related weight losses of about 0.3–1.0 kg for a single ordinary session, with more lost in hotter rooms, in longer sessions, in humid air (where sweat drips without evaporating), and in bathers who are already heat-acclimatised and therefore sweat more profusely. Because the heart and the skin circulation deliver the heat that the sweat removes, the demand for fluid rises in step with the cardiovascular work described under Cardiovascular effects of sauna and Heart rate in a sauna.
Not all of the lost water is replaced between sessions by meals and drinks, particularly when sessions are repeated on the same day or when the bather starts already short of fluid. Dehydration is therefore best understood as a balance over hours, not a single-session event: the morning weight of a person bathing daily can drift downward if drinking does not keep pace. The practical guidance in wellness facilities — drink before, during and after, and take water into the hot room — is aimed precisely at this balance (see Hydration during sauna and the water bottle article).
Effects on the body
A fluid deficit acts on the sauna bather in four connected ways. First, it reduces blood volume, which makes it harder for the circulation to fill the dilated skin vessels, so the heart rate rises more for the same heat load and the characteristic fall in blood pressure in the recovery phase is exaggerated. Second, it impairs sweating: below a certain level of body water, sweat rate declines and the body's main line of cooling weakens, so Core body temperature rises faster. Third, it concentrates the blood and makes the kidneys work harder to conserve water; the small acute shifts in kidney blood flow during heat exposure (see Blood circulation) become more demanding when fluid is short. Fourth, it interacts with Electrolyte balance: ordinary sessions cost mainly water, but sweat also contains sodium, and very heavy, prolonged sweating can deplete salts as well, which matters for the rare but real cases of heat-related illness after long stays.
Acutely, a deficit equivalent to a few per cent of body weight is enough to raise the pulse, dull the sensation of thirst further, and impair mental sharpness. These changes are reversible with drinking and are the reason that symptoms such as dizziness, headache, dark urine and a dry mouth are the conventional warnings (see also Headache and sauna and Fainting in a sauna). If a bather continues bathing despite the deficit, the progression leads toward Heat exhaustion and, in extreme cases, Heat stroke, conditions whose criteria and management are set out on their own pages.
Risk groups and interactions
Some bathers arrive at the sauna already dehydrated or with a reduced ability to conserve water. Older adults often have a weaker thirst sensation and reduced kidney concentrating capacity; diabetes both increases fluid losses through high blood sugar and can conceal them; kidney disease limits the body's ability to regulate water and salts; and medication that promotes urine production — diuretics, including some blood-pressure medicines — raises the baseline fluid need. Alcohol acts as a diuretic and clouds judgement, which is one of the main reasons its combination with bathing is discouraged. The Medical advice article collects the general rule: anybody in these groups, or taking any medication, should discuss sauna bathing with a clinician and should be particularly careful with fluid intake.
Prevention and recognition
The conventional recommendations are simple and are stated in the guidance of public-health authorities: drink water regularly so that urine remains pale; drink extra before and after sweating; replace fluid gradually rather than all at once; and treat dark urine, dizziness, weakness and a racing pulse as signs that fluid is short. In the sauna setting, water should be available in or immediately beside the hot room, and sessions should be shortened when the bather has been drinking little. Rehydration after a session normally involves food as well as drink, because food supplies the sodium that the diet must restore (see Eating after sauna and Food and drink in a wellness facility).
It is worth emphasising what dehydration is not. Losing a kilogram of water weight in a session is not "fat loss", and the rapid fall on the scales after bathing is regained with the next meal (see Weight loss and sauna). Nor is intentional dehydration a health practice: spending extra time in the sauna to "sweat off" water, or combining bathing with sauna clothing designed to increase sweating, raises exactly the risks described above and contributes nothing that ordinary drinking does not provide.
Further reading
References
- Cheuvront SN, Kenefick RW. "Dehydration: physiology, assessment, and performance effects." Comprehensive Physiology 4(1):257–285, 2014. doi:10.1002/cphy.c130017
- Kukkonen-Harjula K, Kauppinen K. "Health effects and risks of sauna bathing." International Journal of Circumpolar Health 65(3):195–205, 2006. doi:10.3402/ijch.v65i3.18102
- Casa DJ, DeMartini JK, Bergeron MF, et al. "National Athletic Trainers' Association position statement: exertional heat illnesses." Journal of Athletic Training 50(9):986–1000, 2015. doi:10.4085/1062-6050-50.9.07
- Laukkanen JA, Laukkanen T, Kunutsor SK. "Cardiovascular and other health benefits of sauna bathing: a review of the evidence." Mayo Clinic Proceedings 93(8):1111–1121, 2018. doi:10.1016/j.mayocp.2018.04.008
- Heinonen I, Laukkanen JA. "Effects of heat and cold on health, with special reference to Finnish sauna bathing." American Journal of Physiology – Regulatory, Integrative and Comparative Physiology 314(5):R629–R638, 2018. doi:10.1152/ajpregu.00115.2017
- Hussain J, Cohen M. "Clinical effects of regular dry sauna bathing: a systematic review." Evidence-Based Complementary and Alternative Medicine 2018:1857413, 2018. doi:10.1155/2018/1857413
- National Health Service. "Dehydration." NHS, UK. nhs.uk/conditions/dehydration (page reviewed 1 May 2026)