Sauna is not simply a tradition. It is a brief encounter with heat as a biological trigger. And just like anything biological, it depends on timing.

The Temperature Clock

We tend to think of temperature objectively. The air is warm. The shower is hot. The weather is cool.

Physiology disagrees. Temperature is an organising force, influencing circulation, metabolism, and the nervous system’s decision to stay vigilant or let go. The body’s circadian rhythm is not only a time-keeper of light and hormones; it also informs the circadian rhythm of body temperature (CRBT), with core temperature rising through the day, tending to fall after sunset.[1]

A sauna session asks your system to carry more heat than it would choose. Peripheral blood flow increases. Sweating begins. Heart rate climbs as cardiac output redirects toward the body surface to dissipate heat.[2] In controlled observations of Finnish sauna exposure, haemodynamic and hormonal changes followed a consistent logic: heat triggers a transient stress response, followed by recovery.[3]

This challenge-and-recovery sequence is what makes sauna beneficial.

Vascular Feedback

Heat reshapes circulation in real time. Peripheral vessels dilate to offload heat, altering blood pressure during and after the session.[2] With repeated exposure, the cardiovascular system adapts similarly to exercise conditioning, though the stimulus is thermal, not mechanical. The cardiovascular system recognises heat as a training-like demand on fluid balance, vascular tone, and autonomic control.

Long-term observational evidence supports this. In a large Finnish study, frequent sauna bathing was associated with lower risks of cardiovascular disease and all-cause mortality.[4] Whereas, a later study including men and women reported similar associations and suggested that sauna habits improved cardiovascular risk prediction.[5]

Greater heat exposure drives stronger cardiovascular adaptation: a classic hormetic dose-response.

Cooling Into Sleep: Why Timing Matters

Sleep is a whole-body state change, not just a psychological event and temperature is central to this shift.

Sleep onset typically coincides with declining core temperature and increased skin heat loss. Efficient heat dissipation smooths the transition into non-REM sleep. Trapped heat (from a warm room, heavy bedding, or poor airflow) fragments sleep and reduces deep sleep and REM.[6]

This is where sauna timing becomes design, not preference.

Warming up before bed can help your body cool down and improve your sleep. A well-known study found that taking a warm bath before bedtime changed sleep patterns in the brain, making it easier to fall asleep when there was enough time between the bath and bedtime.[7] Research confirms that temperature plays a key role: warming up followed by cooling down helps you feel sleepy, while staying too warm can disrupt your sleep quality.[8]

Studies looking at infrared exposure in the early evening offer an interesting insight. When people were exposed to infrared light, their core body temperature and heart rate increased during the session. But their body temperature and heart rate did not drop as early in the night as usual - they dropped later. This suggests that heat exposure can influence your body's internal clock, even without affecting melatonin.[12]

In other words: sauna can have a bigger effect on your sleep-wake cycle than hormones alone.

Morning or Evening?

Sauna culture often divides the day into two stories: morning for energy, evening for rest. But is this scientifically sound?

Morning sessions: activation with intention

In the morning, physiology is already moving toward wakefulness. Core temperature begins to rise. Cortisol secretion is naturally higher. The nervous system is primed for action. A sauna layered onto that state can feel clarifying, even invigorating, largely because it amplifies circulation and sympathetic arousal.[2][3]

Evening sessions: fine tuning your wind-down

Evening sauna has a stronger mechanistic argument when sleep is the goal. The aim is not to go to bed hot.

The aim is to use warmth as a prelude to cooling. For many people, finishing a session roughly 1–3 hours before bedtime allows skin blood flow and heat loss to do their quiet work, aligning with the body's natural nighttime temperature trajectory.[6][8]

Importantly, this is not universal. Some individuals find late heat exposure overstimulating. Others cool quickly and fall asleep more easily. Heat sensitivity, hydration status, and ambient bedroom temperature all matter.

Infrared Sauna: Gentler Heat, Same Benefits

Infrared and far-infrared (FIR) saunas deliver a more refined thermal stimulus than traditional Finnish protocols, with lower ambient heat, deeper tissue penetration, and targeted systemic adaptation.

One peer-reviewed study noted potential cardiovascular benefits from FIR exposure, including blood pressure modulation and as an adjuvant in the treatment of congestive heart failure, as well as evidence pointing to FIR sauna use as a promising therapy for the treatment of chronic pain.[9]

Autonomic effects are where it gets interesting. One study documented shifts in heart rate variability post-FIR exposure, suggesting a parasympathetic tilt: your body recalibrating toward recovery mode.[10]

In a randomized crossover study in 16 male basketball players, a single post-exercise infrared sauna session was compared with seated passive recovery.[11] After a standardized resistance session, athletes completed 20 minutes in an infrared sauna with a measured air temperature of 43 ± 5 °C, or 20 minutes of passive recovery at room temperature.

The infrared sauna condition attenuated the drop in countermovement-jump performance across the recovery window. Muscle soreness ratings were also lower after infrared sauna immediately after the recovery period and the next morning.[11]

Physiologically, heart rate was higher and several HRV indices were lower during the infrared sauna session than during passive recovery, but night-time heart rate, night-time HRV, and self-reported sleep quantity and quality did not differ between conditions.[10]

Taken together, these studies support infrared sauna as a practical recovery adjunct for some aspects of next-day readiness after resistance training, especially explosive performance and perceived soreness.

Infrared & Melatonin

You've probably heard the claim that sauna "boosts melatonin".

Melatonin (your endogenous sleep signal) is primarily driven by light exposure and circadian phase. Temperature can interact with circadian-regulated outputs, but the relationship is context-dependent.

In a study on the effects of infrared radiation on the diurnal rhythms of melatonin, seven healthy young men were exposed to infrared radiation from late afternoon into the early night. Salivary melatonin was sampled hourly, while rectal temperature and heart rate were recorded continuously.[12] Under infrared exposure, rectal temperature and heart rate rose during the exposure window, and the timing of their nightly temperature and heart rate minima shifted later in the night. Importantly, infrared exposure did not affect melatonin concentration in this study.

What does this tell us?

• Sauna supports sleep readiness primarily through warming followed by post-session cooling and shifts in autonomic state.
• Infrared exposure did not affect melatonin concentration, though it did shift the timing of temperature and heart rate rhythms.

A Ritual, Made Intelligent

If sauna is part of your lifestyle, timing is one of the simplest ways to make it work with your biology, rather than against it.

The science-backed way to use it:

• If sleep is the priority, aim to finish your session 1–3 hours before bed. That window gives your body time to cool, which is a key part of the wind-down sequence.[5][7]
• If focus and consistency are the priority, a morning session can be a practical anchor, especially when evenings are busy.

The takeaway is uncomplicated: use heat as a deliberate stressor, then give your system space to recover. Rhythm first. Results follow.

BON CHARGE

This content is for general education and is not medical advice. Our products are not intended to diagnose, treat, cure, or prevent any disease. Always follow product instructions and consult a qualified healthcare professional for guidance tailored to you. Individual results may vary.

References

1. Coiffard, B. et al. A tangled threesome: circadian rhythm, body temperature variations, and the immune system. Biology 10, 65 (2021).
2. Hannuksela, M. L. & Ellahham, S. Benefits and risks of sauna bathing. American Journal of Medicine 110, 118–126 (2001).
3. Kukkonen-Harjula, K. et al. Haemodynamic and hormonal responses to heat exposure in a Finnish sauna bath. European Journal of Applied Physiology 58, 543–550 (1989).
4. Laukkanen, J. A. et al. Association between sauna bathing and fatal cardiovascular and all-cause mortality events. JAMA Internal Medicine 175, 542–548 (2015).
5. Laukkanen, T. et al. Sauna bathing is associated with reduced cardiovascular mortality and improves risk prediction in men and women. BMC Medicine 16, 219 (2018).
6. Okamoto-Mizuno, K. & Mizuno, K. Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology 31, 14 (2012).
7. Horne, J. A. & Reid, A. J. Night-time sleep EEG changes following body heating in a warm bath. Electroencephalography and Clinical Neurophysiology 60, 154–157 (1985).
8. Harding, E. C. et al. The temperature dependence of sleep. Frontiers in Neuroscience 13, 336 (2019).
9. Beever, R. Far-infrared saunas for treatment of cardiovascular risk factors. Canadian Family Physician 55, 691–696 (2009).
10. Lin, Y.-J. et al. Effects of far-infrared radiation on heart rate variability. Lasers in Medical Science 29, 295–301 (2014).
11. Ahokas, E. K. et al. A post-exercise infrared sauna session improves recovery. Biology of Sport 40, 681–689 (2023).
12. Griefahn, B. et al. Effects of infrared radiation on diurnal rhythms of melatonin. Occupational Ergonomics 6, 47–53 (2006).