Far Infrared Radiation (FIR) therapy has gained significant popularity as a non-invasive treatment modality for various health conditions. While the immediate effects of FIR exposure are well-documented, the persistent physiological changes that continue after treatment sessions are less understood.

This review examines the scientific evidence for prolonged effects of FIR on human physiology, focusing on circulatory, metabolic, detoxification, immune, and cellular repair mechanisms that continue to function hours or days after FIR exposure has ceased. Understanding these extended therapeutic effects provides valuable insights into optimizing FIR treatment protocols and maximizing health benefits.

Far Infrared Radiation (FIR), comprising electromagnetic waves in the 4-1000 μm wavelength range, interacts with the human body primarily through resonance with water molecules in tissues. [22] Unlike near-infrared radiation, which directly stimulates mitochondrial cytochrome c oxidase, FIR's thermal resonance effects trigger a cascade of physiological responses that begin during exposure but often continue long after the session has ended.

The immediate benefits of FIR therapy—including peripheral vasodilation, increased microcirculation, pain reduction, and muscle relaxation—are well established. [1] However, growing evidence suggests that FIR exposure initiates longer-lasting physiological processes that contribute to its therapeutic effects. This review synthesizes current research on these persistent effects to provide a comprehensive understanding of FIR's extended influence on human health.

One of the most notable extended effects of FIR therapy involves the cardiovascular system. During FIR exposure, peripheral vasodilation occurs due to thermal effects on vascular smooth muscle and increased nitric oxide (NO) production. [12] Research indicates that this vasodilation persists for up to 24 hours post-treatment, maintaining improved microcirculation to tissues. [14]

Studies in populations with cardiovascular dysfunction have reported improved endothelial function and reduced oxidative stress that persisted for 48 hours after treatment, attributed to increased NO bioavailability and decreased markers of endothelial dysfunction. [10]

The lasting enhancement of blood flow facilitates continued delivery of oxygen and nutrients to tissues while supporting the removal of metabolic waste products. Regular FIR sauna sessions have also been associated with improvements in flow-mediated vasodilation lasting beyond a single exposure window. [9]

FIR exposure initiates a temporary increase in core body temperature, triggering physiological responses similar to those observed during mild fever or exercise. This “artificial fever” can stimulate an increase in metabolic rate that persists beyond the treatment period. [7]

Research has reported that a single FIR sauna session may increase resting metabolic rate for hours post-exposure. [17] This elevated metabolism contributes to increased caloric expenditure and may partly explain weight-management associations seen with regular FIR use.

FIR exposure also appears to influence glucose metabolism. In studies of people with type 2 diabetes, regular FIR therapy was associated with improved fasting blood glucose levels that remained stable for up to 24 hours following treatment, potentially due to enhanced insulin sensitivity linked to improved microcirculation in skeletal muscle tissue. [2]

FIR-induced sweating represents a significant pathway for eliminating compounds from the body. Some reports suggest FIR may promote a sweat profile with higher concentrations of certain heavy metals and fat-soluble compounds. [5] [19]

What’s particularly noteworthy is that mobilization and elimination may continue after the FIR session ends. In one report, higher-than-baseline levels of toxic elements were documented through urine for up to 48 hours post-treatment, suggesting a prolonged detoxification process after external heat application has ceased. [5]

The mechanisms proposed for this extended effect include persistent vasodilation in adipose tissue, which may facilitate continued mobilization of fat-soluble compounds stored in fat cells—allowing gradual elimination through sweat, urine, and feces. [5] This may explain why some individuals report continued sweating at rest for hours after FIR exposure.

Evidence suggests that FIR therapy can have prolonged effects on immune function. During exposure, mild hyperthermia may trigger heat shock protein (HSP) expression—particularly HSP70—associated with protective and immunomodulatory effects. These proteins have been reported to remain elevated for up to 48 hours post-treatment. [20]

Regular FIR sauna use has also been associated with reductions in systemic inflammation markers that persisted beyond the final session window. [11] [8] Additionally, FIR exposure has been reported to stimulate natural killer (NK) cell activity, with increased activity observed for up to 24 hours following treatment in some data. [20]

Some of the most interesting extended effects described in FIR research relate to regeneration processes. FIR exposure may trigger a mild cellular stress response that activates hormetic adaptive mechanisms—where a mild stressor promotes beneficial adaptation that continues after exposure ends. [22]

Research has reported increased mitochondrial biogenesis after FIR exposure, with new mitochondria formation continuing for up to 72 hours post-treatment. [6] FIR has also been associated with reduced oxidative stress markers and elevated antioxidant enzyme activity for up to 24 hours following a session. [6]

FIR exposure has additionally been linked to increased signaling involved in angiogenesis (new blood vessel formation), with continued activity reported for multiple days after treatment in some work—providing a proposed explanation for gradual tissue healing effects with consistent use. [18]

The analgesic effects of FIR therapy have been reported to outlast the treatment session itself. Continued pain relief for up to 48 hours has been described, with several mechanisms proposed: [23]

1. Persistent reductions in muscle tension due to improved microcirculation [14]
2. Continued degradation of pain-inducing metabolites (e.g., lactic acid) through enhanced blood flow [14]
3. Extended modulation of pain signaling pathways, including altered substance P levels [23]
4. Sustained effects on inflammatory cytokine production [20]

The effects of FIR on muscle recovery have also been reported to persist for days. In a study of athletes, FIR therapy after intense exercise was associated with faster recovery of muscle strength and reduced soreness that continued improving for up to 72 hours post-treatment. [15]

FIR therapy appears to have lasting effects on autonomic nervous system function. During exposure, parasympathetic activity typically increases, promoting relaxation and stress reduction. This shift toward parasympathetic dominance has been reported to persist for hours or even days following treatment. [8] [22]

Heart rate variability (HRV) parameters associated with parasympathetic activity have been reported to remain elevated for up to 24 hours following FIR exposure in some work—suggesting a prolonged state of autonomic balance favoring rest and recovery. This extended regulation may contribute to stress-reduction and sleep-improvement benefits commonly reported by FIR users. [8]

These autonomic effects have also been described as dose-dependent in some findings, with longer FIR sessions associated with more persistent parasympathetic activation. [8]

The extended physiological effects of FIR therapy have significant implications for clinical practice and treatment protocols. Understanding that many therapeutic benefits continue or even intensify after the treatment session can inform how sessions are scheduled for maximum benefit.

For detoxification purposes, reports of continued mobilization and excretion for up to 48 hours suggest supporting elimination pathways during this extended period may enhance outcomes (hydration, light movement, and nutritional support are commonly discussed complements). [5]

For cardiovascular applications, the persistence of improved vasodilation and endothelial function for 24–48 hours suggests spacing sessions every 2–3 days may help maintain continuity of vascular benefit. [14] [10] For pain management, extended effects lasting up to 48 hours also align with 2–3 sessions per week in chronic conditions described in the literature. [23]

The lasting effects on immune function and autonomic balance suggest FIR therapy may be especially supportive during periods of stress, illness recovery, or intensive physical training—when recovery signaling is crucial. [20] [8]