Knowledge base · Sleep
It is not the heat that lets you sleep, but what comes after.
A great deal gets claimed about sauna and sleep. We went through the research and set out what was actually measured, why it works the way it does, and where the evidence stops.
The facts first
What actually changes in your sleep after forty
Not shallower, but more broken up. That distinction often gets reversed.
If you notice around fifty that you sleep worse, the feeling is real. It just works differently from the way it is usually told, because most of the loss of deep sleep is behind you by then.
In a study of 149 healthy men aged 16 to 83, published in JAMA in 2000, the share of deep sleep fell from almost 19 per cent among participants aged 16 to 25 to just over 3 per cent in the group aged 36 to 50. Between that middle age and the oldest participants, deep sleep did not decline appreciably any further. Most of it had gone before people usually start to notice.
What does change after that age is how often you wake. In the same study, the time spent awake during the night rose by roughly 28 minutes per decade. A meta-analysis in Sleep from 2004, drawing together 65 studies and 3,577 people, points the same way: time awake after falling asleep is the measure that shifts most with age. Your night does not get shallower, it gets more interrupted.
How it works
Your skin does the work, not the heat
Shedding heat through hands and feet goes with falling asleep sooner.
Your body temperature follows a daily rhythm. It reliably begins to fall about two hours before you fall asleep, and the first episode of deep sleep usually coincides with the steepest part of that decline. That comes from a review article in Frontiers in Neuroscience from 2019.
To shed heat, your body opens the blood vessels in your hands and feet. More blood flows to the skin, and through that skin the heat leaves. This is why warm feet and sleepiness go together, and why a cool bedroom with warm feet sleeps better than the other way round.
A Swiss research group published an analysis of eighteen participants in Nature in 1999. Of everything they measured, the temperature difference between the hands and feet and the rest of the body predicted best how quickly someone fell asleep. The speed at which core temperature itself fell did not predict it.
A misunderstanding worth correcting
You will often read that your body cooling down sets sleep in motion. It is not that simple. The 2019 review states that cooling in itself has not been shown to initiate deep sleep, and is rather a consequence of the blood vessels opening.
In practice this changes little, since warming up and then shedding that heat does go with falling asleep more easily. For the accuracy of the story it matters, which is why it is here.
The evidence
What was actually measured, and on whom
Solid research on bathing, self-reporting on the sauna.
A distinction belongs here that often gets lost. The best research on passive body heating and sleep concerns baths and showers, not the sauna. What is known about the sauna itself comes from questionnaires. Both are set out below, with their origin attached.
Age and sleep
65 studies
The meta-analysis in Sleep from 2004 brought together 65 sleep studies with 3,577 participants aged 5 to 102. Time awake after falling asleep proved the measure that changes most with age.
Baths and showers
17 studies
A meta-analysis in Sleep Medicine Reviews from 2019 reviewed 17 studies. A bath or shower at 40 to 42.5 degrees Celsius, one to two hours before bed, went with falling asleep sooner and better sleep quality. Ten minutes already proved enough.
The mechanism
Two hours ahead
Core temperature starts to fall roughly two hours before sleep onset, and the first deep sleep usually falls in the steepest part of that decline. Frontiers in Neuroscience, 2019.
Sauna users themselves
83.5%
In the Global Sauna Survey of 2019, 83.5 per cent of 482 participants reported sleeping better after sauna use. This is self-reporting by people who signed up of their own accord, without a control group and without any sleep being measured.
In practice
When to take your sauna
The timing from the bathing research is the best guide there is.
Below is what the research on passive body heating points to. It comes from studies of baths and showers. For the sauna that is a reasonable translation rather than a proven prescription, so read it as direction and not as a recipe.
- One to two hours before bed. That is the timing at which the bathing research saw the clearest association. Immediately before bed works less well, because your body still has to shed the heat it has gained.
- It does not need to be long. In the same meta-analysis, ten minutes was already enough to show a difference. For the sauna, the main thing is to stop well before it starts to feel like hard work.
- Take the cooling down seriously. Shedding heat is the whole point. Give yourself time to cool down calmly afterwards rather than climbing straight under a thick duvet.
- Give it a few weeks and see. The results in this research are averages across groups of people. Whether it works for you only shows once you have done it on set evenings for a while.
The limits of the evidence
Three questions we do not skip
What has not been shown here is stated just as plainly.
Has this been studied with the sauna itself?
Barely. For the warm bath there are studies in which sleep was genuinely measured with brain recordings, although the groups were small, a handful of participants each. For the sauna, research of that kind is almost entirely absent. The mechanism is the same, passive heating followed by heat release, but that is a line of reasoning rather than a measurement.
How solid is that 83.5 per cent?
Less solid than the figure looks. Participants filled in a questionnaire themselves, had signed up on their own initiative and were mostly enthusiasts already. There was no control group and nobody’s sleep was measured. A result like that says something about what people experience, not about what happens during their night.
Does a sauna just before bed work as well?
The research does not suggest so. The review literature indicates that heating well before bedtime goes with falling asleep sooner, and heating immediately beforehand does not. Your body needs time to shed the heat again, and you have to allow it that time.
Safety
When to call your doctor first
Heart conditions, pregnancy or sleep medication: talk it through first.
A sauna asks work of your heart and blood vessels. If you have a heart condition or unstable blood pressure, or if you are pregnant, put the question to your doctor or specialist first. The same applies if you take sleep medication or medicines that affect your blood pressure or fluid balance.
If you have been sleeping badly for a longer period, a sauna is no answer to the question of why. Persistent insomnia, snoring with pauses in breathing, or extreme daytime tiredness belong with your doctor. Do not drink alcohol around sauna use, as it disrupts your sleep and raises the risk of overheating. Drink water afterwards.
Further reading
Three articles on heat and habit
Is a hot tub good for your heart?
The Japanese cohort on almost daily warm bathing, with the outcomes for heart and blood vessels and the limits of that evidence.
Will I actually use my spa?
What behavioural research says about keeping it up, and which fixed moment in the week makes the difference.
What a sauna does not do
Seven common claims checked, so you know which effects have and have not been demonstrated.
Accountability
Sources and author
Every figure on this page was checked against the original publication.
See for yourself
A sauna that fits into your evening.
Whether a sauna works for you depends mostly on whether you actually switch it on midweek. That comes down to where it stands in the garden, how long it takes to heat, and which type you choose. We are happy to talk it through with you in our showroom in ’s-Graveland.